Age-Related Physiological and Pathological Changes
Aging is defined, in the perspective of phenotype, the progressive deterioration of physiological function. 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.
Empirical observations on aging have become so numerous and abundant that a special encyclopedia, The Macmillan Encyclopedia of Aging, is now required for even a partial coverage of the accumulated facts.