Period in the human life span in which full physical and intellectual maturity have been attained. Adulthood is commonly thought of as beginning at age 20 or 21. It includes middle age (commencing around age 40) and old age (from about age 60). Physically, it is characterized by the peaking (around age 30) and gradual decline of bodily functioning; the postpeak phase includes diminished acuity of the senses, reduction in muscular and skeletal mass, buildup of cholesterol in the arteries, weakening of the heart muscle, and diminished production of hormones. Some slowing in the rate of central-nervous-system processing also begins with middle age, but it is generally compensated for by an increased capacity to retain practical information and apply accumulated cultural knowledge. In old age, most individuals experience a significant decline in physical capacity, and many eventually also suffer impaired mental function.

This entry comes from Encyclopædia Britannica Concise.
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