A brief treatment of development during adulthood follows. For full treatment, see human development and human behaviour.

Physically, early and middle adulthood are marked by slow, gradual declines in body functioning, which accelerate as old age is reached. The muscle mass continues to increase through the mid-20s, thereafter gradually decreasing. The skeletal mass increases until age 30 or so, and then begins to decrease, first in the central skeleton (pelvis and spine) and last in the peripheral skeleton (fingers and toes). Throughout adulthood there is a progressive deposition of cholesterol in the arteries, and the heart muscle eventually grows weaker even in the absence of detectable disease. The production of both male and female hormones also diminishes with age, though this cannot be directly related to the gradual diminution in sexual activity that occurs in both males and females between 20 and 60.

There is clear evidence that with increasing age adults display a slow, very gradual tendency toward decreasing speed of response in the execution of intellectual (and physical) tasks. Slowing rates of electrical activity in the older adult brain have been linked to the slowing of behaviour itself. This decline in the rate of central nervous system processing does not necessarily imply similar changes in learning, memory, or other intellectual functions. The learning capacity of young adults is superior to that of older adults, as is their ability to organize new information in terms of its content or meaning. Older adults, on the other hand, are equal or superior to young adults in their capacity to retain general information and in their accumulated cultural knowledge. See also aging; old age.