Physical changes that occur during adulthood include changes in body shape, muscular development, sexual activity, and, eventually, the loss of body functions because of aging.

Adulthood is the human life stage in which an individual has reached full physical and mental maturity, typically starting at 20 years and ending at death. Adulthood is often divided into categories: young adults, ages 20 to 40 years; middle-aged adults, 40 to 65 years; and older adults, 65 years and beyond.

Many physical changes occurring during adulthood involve degeneration. Young adults up to about age 30 reach physical and intellectual peaks. Young adults over 30 years and middle-aged adults experience gradual declines, and older adults undergo rapid decline. These changes, which are most visible in older adults, are part of the varied processes associated with aging. Muscle mass begins to decrease after its peak in the mid-20s. Bone (skeletal) mass peaks at about age 30, before beginning to decline. This degeneration begins in the pelvis and spine, the central skeleton, gradually moves throughout the skeleton, and finally reaches the fingers and toes. Consequently, adults begin losing height at middle age and may experience weight gain.

Many other changes during adulthood affect other parts of the body, including the cardiovascular and endocrine systems. Cholesterol can gradually accumulate in the arteries. Heart muscles grow weaker, even in healthy adults. Arteries and arterioles lose elasticity with aging, so older adults are more likely to exhibit hypertension (high blood pressure). Male and female hormone production drops with age, and females undergo menopause, the ceasing of menstruation, which generally occurs in middle age.