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What is a Aging?
Definitions of aging differ between biologists and behavioral scientists. Biologists regard aging as reflecting the sum of multiple and typical biological decrements occurring after sexual maturation (or possibly well before that, such as time of birth or even pre-natal stage). Behavioral scientists view it as reflecting regular and expected changes occurring in genetically-representative organisms advancing through the life cycle under normal environmental conditions. It is difficult to define normal aging, since many changes observed in older adults and previously perceived as concomitants or normal aging are now recognized as effects disease later in life. The behavioral-science view allows for incremental as well as decremental changes with aging. Senescence is not always equated with aging; it is viewed as the increasing vulnerability or decreasing capacity of an organism to maintain homeostasis as it progresses through its life span. Gerontology refers to the study of aging. Geriatrics refers to the clinical science that is concerned with the health and illness in the elderly.
Inquiries into why organisms age involve both purpose of aging and the process of aging. There are theories, but no conclusive evidence, in both areas. The most-common explanations of the purpose of aging are based on theories that aging is adaptive for a species. Theories about the process of aging concern how people age. These biological theories address two sets factors -- intrinsic and extrinsic to the organism. Intrinsic factors are influences operating on the human body from within, such as the impact of genetic programming. Extrinsic factors are influences on the body from the environment, such as the impact of cumulative stress. Despite an abundance of theories to explain process of aging, the mechanisms remain a mystery.
It is important to differentiate between life expectancy and life span. Life expectancy is the average number of years of life in a given species; it is significantly influenced by factors beyond the aging process alone, such as famine, (mal)nutrition, caloric intake, environment and disease. Life span is the maximum number of years of life possible for that species; it is more fundamentally linked to the aging process itself. Over the centuries, life expectancy (aka mean or average life span) has increased* (due to improved sanitation and health-care practices); maximum life span has not*. Approximately 115 years appears to be the upper limit of life span in humans.
*Calorie restriction is the only intervention known to science that can increase average lifespan and extend maximum lifespan.
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