Aging is the natural process of physical and mental change that occurs over time. It is often associated with changes in physical appearance, such as the development of wrinkles on the skin. Functional and cognitive abilities are also external indicators of aging.

This provides the distribution of the population according to age. Information may be included by sex and age group (0-14 years, 15-64 years, 65 years and over).

The age structure of a population affects a nation's key socioeconomic issues. Countries with young populations (high percentage under age 15) need to invest more in schools, while countries with older populations (high percentage ages 65 and over) need to invest more in the health sector.

The age structure can also be used to help predict potential political issues. For example, the rapid growth of a young adult population unable to find employment can lead to unrest.

This is the age that divides a population into two numerically equal groups; that is, half the people are younger than this age and half are older. It is a single index that summarizes the age distribution of a population.

Currently, the median age ranges from a low of about 15 in Uganda and Gaza Strip to 40 or more in several European countries and Japan. See "Age structure" above for the importance of a young versus an older age structure and, by implication, a low versus a higher median age.