Aging or ageing is the process of becoming older. The term refers especially to humans, many other animals, and fungi. In the broader sense, aging can refer to single cells within an organism which have ceased dividing (cellular senescence) or to the population of a species (population aging).
This article focuses primarily on humans. Aging represents the accumulation of changes in a human being over time and can encompass physical, psychological, and social changes. Reaction time, for example, may slow with age, while memories and general knowledge typically increase. Society's view of aging, for example valuing youth over experience, affects a person's self-perception of their own aging. A belief in an afterlife, the continued existence of an eternal spirit or soul after the death of the physical body, gives a different view to aging and reduces the stress associated with physical deterioration.
Biological aging refers to an organism's increased rate of death as it progresses through its lifecycle and increases its chronological age. Another possible way to define aging is through functional definitions, of which there are two main types: The first describes how varying types of deteriorative changes that accumulate in the life of a post-maturation organism can leave it vulnerable, leading to a decreased ability of the organism to survive: "Aging is the progressive accumulation of changes with time that are associated with or responsible for the ever-increasing susceptibility to disease and death which accompanies advancing age." The second is a senescence-based definition which describes age-related changes in an organism that increase its mortality rate over time by negatively affecting its vitality and functional performance.
An important distinction to make is that biological aging is not the same as the accumulation of diseases related to old age; disease is a blanket term used to describe a process within an organism that causes a decrease in its functional ability. Aging is the natural and inevitable biological process that all animal life, including human beings, go through from conception through birth to death. While death by other external causes, such as disease, accident, predation, and so forth, is common, nonetheless death would occur naturally due to aging even in the absence of such causes.
The causes of aging are uncertain, but appear to involve a number of factors. The factors proposed to influence biological aging fall into two main categories: programmed and damage-related. They are not necessarily mutually exclusive.
The first posits that aging is programmed and therefore follows an inexorable path, a biological timetable, perhaps one that might be a continuation of that which regulates childhood growth and development. This regulation would depend on changes in gene expression that affect the systems responsible for maintenance, repair, and defense responses. Programmed aging should not be confused with programmed cell death (apoptosis).
The second category of theories suggests various sources and targets of damage that lead to aging. Damage-related factors include internal and environmental assaults to living organisms that induce cumulative damage at various levels.
A third, novel concept is that aging is mediated by vicious cycles.
Additionally, there can be other reasons which can speed up the rate of aging in organisms including human beings, such as obesity and compromised immune system.
The programmed approach suggests three major mechanisms which control aging:
- Programmed Longevity
Aging is the result of a sequential switching on and off of certain genes, with senescence being defined as the time when age-associated deficits are manifested.
- Endocrine Theory
Biological clocks act through hormones to control the pace of aging.
- Immunological Theory
The rate of aging varies substantially across different species, and this, to a large extent, is genetically based. For example, numerous perennial plants ranging from strawberries and potatoes to willow trees typically produce clones of themselves by vegetative reproduction and are thus potentially immortal, while annual plants such as wheat and watermelons die each year and reproduce by sexual reproduction. The oldest animals known so far are 15,000-year-old Antarctic sponges, which can reproduce both sexually and clonally.
Clonal immortality apart, there are certain species whose individual lifespans stand out among Earth's life-forms, including the bristlecone pine at around 5,000 years invertebrates like the hard clam (known as quahog in New England) at 508 years, the Greenland shark at 400 years, various deep-sea tube worms at over 300 years, and lobsters. Such organisms are sometimes said to exhibit negligible senescence.
Numerous damage-related factors have been proposed that lead to aging, including the following:
- Wear and tear theory
Cells and tissues have vital parts that wear out resulting in aging. Like components of an aging machine, parts of the body eventually wear out from repeated use, leading to cell death and inability to function.
- Rate of living theory
- Cross-linking theory
According to this theory, an accumulation of cross-linked proteins damages cells and tissues, slowing down bodily processes and resulting in aging.
- Free radical theory
This proposes that free radicals cause damage to the macromolecular components of the cell, giving rise to accumulated damage causing cells, and eventually organs, to stop functioning. Macromolecules, such as nucleic acids, lipids, sugars, and proteins, are susceptible to free radical attack. Enzymes, which are natural antioxidants, are found in the body and function to curb build-up of free radicals.
- Somatic DNA damage theory
DNA damage occurs continuously in cells of living organisms. While most of the damage is repaired naturally, some accumulates as the repair mechanisms cannot correct defects as fast as they are produced. Thus, aging results from damage to the genetic integrity of the body’s cells.
Other suggested factors include: progressive loss of physiological integrity through genomic instability (mutations accumulated in nuclear DNA, in mtDNA, and in the nuclear lamina) and telomere attrition. Also, accumulation of waste products in cells presumably interferes with metabolism. For example, a waste product called lipofuscin is formed by a complex reaction in cells that binds fat to proteins. This waste accumulates in the cells as small granules, which increase in size as a person ages.
In humans, aging represents the accumulation of changes in a human being over time and can encompass physical, psychological, and social changes. Reaction time, for example, may slow with age, while memories and general knowledge typically increase. Aging also increases the risk of diseases.
A number of characteristic aging symptoms are experienced by a significant proportion of human beings during their lifetimes, including the following:
- After peaking in the mid-20s, female fertility declines.
- After age 30 the mass of human body is decreased until 70 years and then shows damping oscillations.
- Muscles have reduced capacity of responding to exercise or injury and loss of muscle mass and strength (sarcopenia) is common.
- Hand strength and mobility are decreased during the aging process. These things include "hand and finger strength and ability to control submaximal pinch force and maintain a steady precision pinch posture, manual speed, and hand sensation."
- People over 35 years of age are at increasing risk for losing strength in the ciliary muscle of the eyes which leads to difficulty focusing on close objects, or presbyopia. Most people experience presbyopia by age 45–50.
- Around age 50, hair turns grey. Pattern hair loss or baldness by the age of 50 affects about 50 percent of men and 25 percent of women.
- Menopause typically occurs between 44 and 58 years of age.
- In the 60–64 age cohort, the incidence of osteoarthritis rises.
- Wrinkles develop mainly due to photoageing, particularly affecting sun-exposed areas (face).
- Almost half of people older than 75 have hearing loss (presbycusis) inhibiting spoken communication.
- By age 80, more than half of all Americans either have a cataract or have had cataract surgery.
- Frailty, a syndrome of decreased strength, physical activity, physical performance and energy, affects 25 percent of those over 85.
- Atherosclerosis is classified as an aging disease, which leads to cardiovascular disease (for example stroke and heart attack), globally the most common causes of death.
- Dementia becomes more common with age. The spectrum ranges from mild cognitive impairment to the neurodegenerative diseases of Alzheimer's disease, cerebrovascular disease, Parkinson's disease and Lou Gehrig's disease. bout 3 percent of people between the ages of 65 and 74, 19 percent between 75 and 84, and nearly half of those over 85 years of age have dementia. Furthermore, many types of memory decline with aging, but not semantic memory or general knowledge such as vocabulary definitions, which typically increases or remains steady until late adulthood. Individual variations in rate of cognitive decline may be explained in terms of people having different lengths of life.
Prevention and delay
Human beings who do not have a belief in an afterlife, the continued existence of an eternal spirit or soul after the death of their physical body, have sought ways to prevent aging and death, or at least to delay the process. Such research supports each individual's likelihood of living their full lifespan in good health. The following are factors which have been found to increase the length and quality of life.
A healthy diet may reduce the effects of aging. For example, the Mediterranean diet is credited with lowering the risk of heart disease and early death. The major contributors to mortality risk reduction appear to be a higher consumption of vegetables, fish, fruits, nuts, and monounsaturated fatty acids (olive oil).
Amount of sleep is related to mortality. People who live the longest report sleeping for six to seven hours each night, while the National Sleep Foundation recommends eight hours of sleep per night for optimal health. However, this range of sleep has not been shown to be causal in increasing life span, merely correlated with longer life which may be affected by various other factors. Studies linking longer or shorter sleep patterns to increased mortality do not necessarily imply that people should change their existing, and comfortable, sleep patterns. 
Physical exercise may increase life expectancy. People who participate in moderate to high levels of physical exercise have a lower mortality rate compared to individuals who are not physically active. Moderate levels of exercise have been correlated with preventing aging and improving quality of life by reducing inflammatory potential. Regular physical exercise has also been shown to have restorative properties for cognitive and brain function in old age.
Avoidance of chronic stress (as opposed to acute stress) is associated with a slower loss of telomeres in most but not all studies, and with decreased cortisol levels, both of which are factors in increasing morbidity and mortality. Stress can be countered by social connection, spirituality, and (for men more clearly than for women) married life, all of which are associated with longevity.
Theoretically, extension of maximum lifespan in humans could be achieved by reducing the rate of aging damage by periodic replacement of damaged tissues, molecular repair or rejuvenation of deteriorated cells and tissues, reversal of harmful epigenetic changes, or the enhancement of enzyme telomerase activity. Research geared towards life extension strategies in various organisms is currently under way at a number of academic and private institutions. Various drugs and interventions have been shown to slow or reverse the biological effects of aging in animal models, but none has yet been proven to do so in humans.
Society and culture
Different cultures express age in different ways. Most legal systems define a specific age for when an individual is allowed or obliged to do particular activities. These age specifications include voting age, drinking age, age of consent, age of majority, age of criminal responsibility, marriageable age, age of candidacy, and mandatory retirement age. In other words, chronological aging may be distinguished from "social aging" (cultural age-expectations of how people should act as they grow older) and "biological aging" (an organism's physical state as it ages).
In the fielda of sociology and mental health, ageing is seen in five different views: aging as maturity, aging as decline, aging as a life-cycle event, aging as generation, and aging as survival. Positive correlates with aging often include economics, employment, marriage, children, education, and sense of control. Retirement, a common transition faced by the elderly, may have both positive and negative consequences.
Population aging is the increase in the number and proportion of older people in society. Population aging occurs through migration, longer life expectancy (decreased death rate), and decreased birth rate. As a population ages, it has a significant impact on society. Young people tend to have fewer legal privileges (if they are below the age of majority), 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, and frequently have differing values as well, such as for property and pension rights.
Among the most urgent concerns of older persons worldwide is income security. This poses challenges for governments with aging populations to ensure that investment in pension systems is sufficient to provide economic independence and reduce poverty in old age. These challenges vary for developing and developed countries:
Sustainability of these systems is of particular concern, particularly in developed countries, while social protection and old-age pension coverage remain a challenge for developing countries, where a large proportion of the labour force is found in the informal sector.
With age inevitable biological changes occur that increase the risk of illness and disability. A number of health problems become more prevalent as people get older. These include mental health problems as well as physical health problems, especially dementia.
While the effects on society of population aging are multidimensional, there is no doubt about its impact on health care demand:
A life-cycle approach to health care – one that starts early, continues through the reproductive years and lasts into old age – is essential for the physical and emotional well-being of older persons, and, indeed, all people. Public policies and programmes should additionally address the needs of older impoverished people who cannot afford health care.
As humans age, their bodies begin to break down, and among other changes their skin begins to look different. However, especially in Western culture, these changes are not always appreciated and being young, or at least youthful, is celebrated especially when young people are successful.
Generally, aversion to aging and its effects is primarily a Western attitude. In other places around the world, old age is celebrated and honored. In Korea, for example, a special party called hwangap is held to celebrate and congratulate an individual for turning 60 years old.
Positive self-perceptions of aging are associated with better mental and physical health and well-being. Positive self-perception of health has been correlated with higher well-being and reduced mortality among the elderly. Various reasons have been proposed for this association; people who are objectively healthy may naturally rate their health better as than that of their ill counterparts.
As people age, subjective health remains relatively stable, even though objective health worsens. In fact, perceived health improves with age when objective health is controlled in the equation. This phenomenon is known as the "paradox of aging," and may be a result of social comparison; for instance, the older people get, the more they may consider themselves in better health than their same-aged peers. Elderly people often associate their functional and physical decline with the normal aging process.
Traditional definitions of successful aging have emphasized absence of physical and cognitive disabilities. Successful aging has been characterized as involving three components: a) freedom from disease and disability, b) high cognitive and physical functioning, and c) social and productive engagement.
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ReferencesISBN links support NWE through referral fees
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