Aging is the accumulation of changes in a person over time. Aging in humans refers to a multidimensional process of physical, psychological, and social change. Some dimensions of aging 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. Aging is an important part of all human societies reflecting the biological changes that occur, but also reflecting cultural and societal conventions. Roughly 100,000 people worldwide die each day of age-related causes.
Age is measured chronologically, and a person\'s birthday is often an important event. However the term "aging" is somewhat ambiguous. Distinctions may be made between "universal aging" (age changes that all people share) and "probabilistic aging". Chronological aging may also 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). There is also a distinction between "proximal aging" (age-based effects that come about because of factors in the recent past) and "distal aging" (age-based differences that can be traced back to a cause early in person\'s life, such as childhood poliomyelitis).
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.
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