Ageing or aging (American English) is the accumulation of changes in an organism or object over time. Ageing in humans refers to a multidimensional process of physical, psychological, and social change. Some dimensions of ageing grow and expand over time, while others decline. Reaction time, for example, may slow with age, while knowledge of world events and wisdom may expand. Research shows that even late in life potential exists for physical, mental, and social growth and development. Ageing is an important part of all human societies reflecting the biological changes that occur, but also reflecting cultural and societal conventions. Age is usually measured in full years -- and months for young children. A person's birthday is often an important event.
The term "ageing" is somewhat ambiguous. Distinctions may be made between "universal ageing" (age changes that all people share) and "probabilistic ageing" (age changes that may happen to some, but not all people as they grow older, such as the onset of Type Two diabetes). Chronological ageing, referring to how old a person is, is arguably the most straightforward definition of ageing and may be distinguished from "social ageing" (society's expectations of how people should act as they grow older) and "biological ageing" (an organism's physical state as it ages). There is also a distinction between "proximal ageing" (age-based effects that come about because of factors in the recent past) and "distal ageing" (age-based differences that can be traced back to a cause early in person's life, such as childhood poliomyelitis).
Differences are sometimes made between populations of children;divisions are sometimes made between the young old (65-74), the middle old (75-84) and the old old (those aged 85 and above).(Beyond this,people differ on identifying "oldest old","super-old",and other names for age brackets). However, problematic in this is that chronological age does not correlate perfectly with functional age, i.e. two people may be of the same age, but differ in their mental and physical capacities.
Population ageing is the increase in the number and proportion of older people in society. Population ageing has three possible causes: migration, longer life expectancy (decreased death rate), and decreased birth rate. Ageing has a significant impact on society. Young people tend to commit most crimes, they are more likely to push for political and social change, to develop and adopt new technologies, and to need education. Older people have different requirements from society and government as opposed to young people, and frequently differing values as well. Older people are also far more likely to vote, and in many countries the young are forbidden from voting. Thus, the aged have comparatively more political influence.
- 1 Senescence
- 2 Dividing the lifespan
- 3 Cultural variations
- 4 Society
- 5 Cognitive effects
- 6 Coping and well-being
- 7 Societal impact
- 8 Terminology
- 9 Theories
- 10 Measure of age
- 11 Notes
- 12 References
- 13 External links
- Main article: Senescence
In biology, senescence is the state or process of aging. Cellular senescence is a phenomenon where isolated cells demonstrate a limited ability to divide in culture (the Hayflick Limit, discovered by Leonard Hayflick in 1965), while Organismal senescence is the aging of organisms. After a period of near perfect renewal (in humans, between 20 and 35 years of age), organismal senescence is characterized by the declining ability to respond to stress, increasing homeostatic imbalance and increased risk of disease. This irreversible series of changes inevitably ends in death. Some researchers (specifically biogerontologists) are treating aging as a disease. As genes that have an effect on aging are discovered, aging is increasingly being regarded in a similar fashion to other genetic conditions, potentially "treatable."
Indeed, aging is not an unavoidable property of life. Instead, it is the result of a genetic program. Numerous species show very low signs of aging ("negligible senescence'), the best known being trees like the bristlecone pine, fish like the sturgeon and the rockfish, invertebrates like the quahog or sea anemone .
In humans and other animals, cellular senescence has been attributed to the shortening of telomeres with each cell cycle; when telomeres become too short, the cells die. The length of telomeres is therefore the "molecular clock," predicted by Hayflick. Telomere length is maintained in immortal cells (e.g. germ cells and keratinocyte stem cells, but not other skin cell types) by the enzyme telomerase. In the laboratory, mortal cell lines can be immortalized by the activation of their telomerase gene, present in all cells but active in few cell types. Cancerous cells must become immortal to multiply without limit. This important step towards carcinogenesis implies, in 85% of cancers, the reactivation of their telomerase gene by mutation. Since this mutation is rare, the telomere "clock" can be seen as a protective mechanism against cancer .
Other genes are known to affect the aging process, the sirtuin family of genes have been shown to have a significant effect on the lifespan of yeast and nematodes. Over-expression of the RAS2 gene increases lifespan in yeast substantially.
In addition to genetic ties to lifespan, diet has been shown to substantially affect lifespan in many animals. Specifically, caloric restriction (that is, restricting calories to 30-50% less than an ad libitum animal would consume, while still maintaining proper nutrient intake), has been shown to increase lifespan in mice up to 50%. Caloric restriction works on many other species beyond mice (including species as diverse as yeast and Drosophila), and appears (though the data is not conclusive) to increase lifespan in primates according to a study done on Rhesus monkeys at the National Institute of Health (US), although the increase in lifespan is only notable if the caloric restriction is started early in life. Since, at the molecular level, age is counted not as time but as the number of cell doublings, this effect of calorie reduction could be mediated by the slowing of cellular growth and, therefore, the lengthening of the time between cell divisions.
Drug companies are currently searching for ways to mimic the lifespan-extending effects of caloric restriction without having to severely reduce food consumption.
Dividing the lifespan
A human life is often divided into various ages. Historically, the lifespan of people are divided into seven ages; because biological changes are slow moving and vary from person to person, arbitrary dates are usually set to mark periods of human life. In some cultures the divisions given below are quite varied.
- Pre-conception: ovum, spermatozoon
- Conception: fertilization
- Pre-birth: conception to birth (pregnancy)
- Infancy: Birth to 2
- Childhood: 2 to 11
- Adolescence: 12 to 19
- Early adulthood: 20 to 40
- Middle adulthood: 40 to 65
- Late adulthood: 65+
- Post-Death: Decomposition
Ages can also be divided by decade:
- Denarian: someone between 10 and 19 years of age
- Vicenarian: someone between 20 and 29 years of age
- Tricenarian: someone between 30 and 39 years of age
- Quadragenarian: someone between 40 and 49 years of age
- Quinquagenarian: someone between 50 and 59 years of age
- Sexagenarian: someone between 60 and 69 years of age
- Septuagenarian: someone between 70 and 79 years of age
- Octogenarian: someone between 80 and 89 years of age
- Nonagenarian: someone between 90 and 99 years of age
- Centenarian: someone over 100 years of age
- Supercentenarian: someone over 110 years of age
In some cultures (for example Serbian and Russian) there are two ways to express age: by counting years with or without including current year. For example, it could be said about the same person that he is twenty years old or that he is in the twenty-first year of his life. In Russian the former expression is generally used, the latter one has restricted usage: it is used for age of a deceased person in obituaries and for the age of a child when it is desired to show him/her older than he/she is. (Psychologically, a boy in his 4th year seems older than one who is 3 years old.)
Considerable numbers of cultures have less of a problem with age compared with what has been described above, and it is seen as an important status to reach stages in life, rather than defined numerical ages. Advanced age is given more respect and status.
East Asian age reckoning is different from that found in Western culture. Traditional Chinese culture uses a different aging method, called Xusui (²) with respect to common aging which is called Zhousui (¨²). In the Xusui method, people are born at age 1, not age 0.
There are variations in many countries as to what age a person legally becomes an adult.
Most legal systems define a specific age for when an individual is allowed or obliged to do something. These ages include voting age, drinking age, age of consent, age of majority, age of criminal responsibility, marriageable age, age where one can hold public office, and mandatory retirement age. Admission to a movie for instance, may depend on age according to a motion picture rating system. A bus fare might be discounted for the young or old.
Similarly in many countries in jurisprudence, the defense of infancy is a form of defense by which a defendant argues that, at the time a law was broken, they were not liable for their actions, and thus should not be held liable for a crime. Many courts recognize that defendants who are considered to be juveniles may avoid criminal prosecution on account of their age, and in borderline cases the age of the offender is often held to be a mitigating circumstance.
Economics and marketing
The economics of aging are also of great import. Children and teenagers have little money of their own, but most of it is available for buying consumer goods. They also have considerable impact on how their parents spend money.
Young adults are an even more valuable cohort. They often have jobs with few responsibilities such as a mortgage or children. They do not yet have set buying habits and are more open to new products.
Health care demand
Many societies in the rich world, e.g. Western Europe and Japan, have aging populations. While the effects on society are complex, there is a concern about the impact on health care demand. The large number of suggestions in the literature for specific interventions to cope with the expected increase in demand for long-term care in aging societies can be organized under four headings: improve system performance; redesign service delivery; support informal caregivers; and shift demographic parameters.
However, the annual growth in national health spending is not mainly due to increasing demand from aging populations, but rather has been driven by rising incomes, costly new medical technology, a shortage of health care workers and informational asymmetries between providers and patients.
Even so, it has been estimated that population aging only explains 0.2 percentage points of the annual growth rate in medical spending of 4.3 percent since 1970. In addition, certain reforms to Medicare decreased elderly spending on home health care by 12.5 percent per year between 1996 and 2000.  This would suggest that the impact of aging populations on health care costs is not inevitable.
Impact on Prisons
As of July 2007, medical costs for a typical inmate might run an agency around $33 per day, while costs for an aging inmate could run upwards of $100. Most DOCs report spending more than 10 percent of the annual budget on elderly care. That is expected to rise over the next 10-20 years. Some states have talked about releasing aging inmates early. 
Steady decline in many cognitive processes are seen across the lifespan, starting in one's thirties. Research has focused in particular on memory and aging, and has found decline in many types of memory with aging, but not in semantic memory or general knowledge such as vocabulary definitions, which typically increases or remains steady. Early studies on changes in cognition with age generally found declines in intelligence in the elderly, but studies were cross-sectional rather than longitudinal and thus results may be an artefact of cohort rather than a true example of decline. Intelligence may decline with age, though the rate may vary depending on the type, and may in fact remain steady throughout most of the lifespan, dropping suddenly only as people near the end of their lives. Individual variations in rate of cognitive decline may therefore be explained in terms of people having different lengths of life.
Coping and well-being
Psychologists have examined coping skills in the elderly. Various factors, such as social support, religion and spirituality, active engagement with life and having an internal locus of control have been proposed as being beneficial in helping people to cope with stressful life events in later life. Social support and personal control are possibly the two most important factors that predict well-being, morbidity and mortality in adults. Other factors that may link to well-being and quality of life in the elderly include social relationships (possibly relationships with pets as well as humans), and health.
Individuals in different wings in the same retirement home have demonstrated a lower risk of mortality and higher alertness and self-rated health in the wing where residents had greater control over their environment, though personal control may have less impact on specific measures of health. Social control, perceptions of how much influence one has over one's social relationships, shows support as a moderator variable for the relationship between social support and perceived health in the elderly, and may positively influence coping in the elderly.
Religion has been an important factor used by the elderly in coping with the demands of later life, and appears more often than other forms of coping later in life. Religious commitment may also be associated with reduced mortality, though religiosity is a multidimensional variable; while participation in religious activities in the sense of participation in formal and organized rituals may decline, it may become a more informal, but still important aspect of life such as through personal or private prayer.
Self-ratings of health, the beliefs in one's own health as excellent, fair or poor, has been correlated with well-being and mortality in the elderly; positive ratings are linked to high well-being and reduced mortality. Various reasons have been proposed for this association; people who are objectively healthy may naturally rate their health better than that of their ill counterparts, though this link has been observed even in studies which have controlled for socioeconomic status, psychological functioning and health status. This finding is generally stronger for men than women, though the pattern between genders is not universal across all studies, and some results suggest sex-based differences only appear in certain age groups, for certain causes of mortality and within a specific sub-set of self-ratings of health.
Societal aging refers to the demographic aging of populations and societies. Cultural differences in attitudes to aging have been studied.
Given the physical and cognitive declines seen in aging, a surprising finding is that emotional experience improves with age. Older adults are better at regulating their emotions and experience negative affect less frequently than younger adults and show a positivity effect in their attention and memory. The emotional improvements show up in longitudinal studiesTemplate:Specify as well as in cross-sectional studiesTemplate:Specify and so cannot be entirely due to only the happier individuals surviving.
The concept of successful aging can be traced back to the 1950s, and popularised in the 1980s. Previous research into aging exaggerated the extent to which health disabilities, such as diabetes or osteoporosis, could be attributed exclusively to age, and research in gerontology exaggerated the homogeneity of samples of elderly people.
Successful aging consists of three components:
- Low probability of disease or disability;
- High cognitive and physical function capacity;
- Active engagement with life.
A greater number of people self-report successful aging than those that strictly meet these criteria.
Successful aging may be viewed an interdisciplinary concept, spanning both psychology and sociology, where it is seen as the transaction between society and individuals across the life span with specific focus on the later years of life. The terms "healthy aging" "optimal aging" have been proposed as alternatives to successful aging.
Six suggested dimensions of successful aging include:
- No physical disability over the age of 75 as rated by a physician;
- Good subjective health assessment (i.e. good self-ratings of one's health);
- Length of undisabled life;
- Good mental health;
- Objective social support;
- Self-rated life satisfaction in eight domains, namely marriage, income-related work, children, friendship and social contacts, hobbies, community service activities, religion and recreation/sports
- Disengagement Theory
- This is the idea that separation of older people from active roles in society is normal and appropriate, and benefits both society and older individuals. Disengagement theory, first proposed by Cumming and Henry, has received considerable attention in gerontology, but has been much criticised. The original data on which Cumming and Henry based the theory were from a rather small sample of older adults in Kansas City, and from this select sample Cumming and Henry then took disengagement to be a universal theory. There are research data suggesting that the elderly who do become detached from society as those were initially reclusive individuals, and such disengagement is not purely a response to aging.
- Activity Theory
- In contrast to disengagement theory, this theory implies that the more active elderly people are, the more likely they are to be satisfied with life. The view that elderly adults should maintain well-being by keeping active has had a considerable history, and since 1972, this has become to be known as activity theory. However, this theory may be just as inappropriate as disengagement for some people as the current paradigm on the psychology of aging is that both disengagement theory and activity theory may be optimal for certain people in old age, depending on both circumstances and personality traits of the individual concerned. There are also data which query whether, as activity theory implies, greater social activity is linked with well-being in adulthood.
- Selectivity Theory - mediates between Activity and Disengagement Theory, which suggests that it may benefit older people to become more active in some aspects of their lives, more disengaged in others.
- Continuity Theory
- The view that in aging people are inclined to maintain, as much as they can, the same habits, personalities, and styles of life that they have developed in earlier years. Continuity theory is Atchley's theory that individuals, in later life, make adaptations to enable them to gain a sense of continuity between the past and the present, and the theory implies that this sense of continuity helps to contribute to well-being in later life. Disengagement theory, activity theory and continuity theory are social theories about ageing, though all may be products of their era rather than a valid, universal theory.
- Telomere Theory
- Telomeres (structures at the ends of chromosomes) have experimentally been shown to shorten with each successive cell division. Shortened telomeres activate a mechanism that prevents further cell multiplication. This may be an important mechanism of aging in tissues like bone marrow and the arterial lining where active cell division is necessary.
- Reproductive-Cell Cycle Theory
- The idea that aging is regulated by reproductive hormones that act in an antagonistic pleiotropic manner via cell cycle signaling, promoting growth and development early in life in order to achieve reproduction, but later in life, in a futile attempt to maintain reproduction, become dysregulated and drive senescence (dyosis).
- Wear-and-Tear theory
- The idea that changes associated with aging are the result of chance damage that accumulates over time.
- Somatic Mutation Theory
- The biological theory that aging results from damage to the genetic integrity of the body's cells.
- Error Accumulation Theory
- The idea that aging results from chance events that escape proof reading mechanisms, which gradually damages the genetic code.
- Evolutionary Theories
- See Theories of aging in Senescence. These have by far the most theoretical and empirical support.
- Accumulative-Waste Theory
- The biological theory of aging that points to a buildup of cells of waste products that presumably interferes with metabolism.
- Autoimmune Theory
- The idea that aging results from an increase in autoantibodies that attack the body's tissues. A number of diseases associated with aging, such as atrophic gastritis and Hashimoto's thyroiditis, are probably autoimmune in this way.
- Aging-Clock Theory
- The theory that aging results from a preprogrammed sequence, as in a clock, built into the operation of the nervous or endocrine system of the body. In rapidly dividing cells the shortening of the telomeres would provide just such a clock.
- Cross-Linkage Theory
- The idea that aging results from accumulation of cross-linked compounds that interfere with normal cell function.
- Free-Radical Theory
- The idea that free radicals (unstable and highly reactive organic molecules, also named reactive oxygen species or oxidative stress) create damage that gives rise to symptoms we recognize as aging.
- It has been known since the 1930s that restricting calories while maintaining adequate amounts of other nutrients prevents aging across a broad range of organism. Recently, Michael Ristow has shown that this delay of aging is due to increased formation of free radicals within the mitochondria causing a secondary induction of increased antioxidant defence capacity.
Measure of age
The age of an adult human is commonly measured in whole years since the day of birth. Fractional years or months or even weeks may be used to describe the age of children and infants for finer resolution. The time of day the birth occurred is not commonly considered.
The measure of age has historically varied from this approach in some cultures. For example, in China and Japan, children were considered to be one year old at the moment of birth and two years old on the following Near Year's day. Thus, a child could be considered two years old several days after birth.
Age in prenatal development is normally measured in gestational age, taking the last menstruation of the woman as a point of beginning. Alternatively, fertilization age, beginning from fertilization can be taken.
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