America’s population is growing because more people are being born than are dying and because immigrants, most in their late teens or early 20s, are still coming to the United States. This combination means that the American population is younger than in other developed nations. In 2001, 21 percent of the population in the United States was under the age of 15 This compares with 18 percent in Europe and 15 percent in Japan. Because the U.S. population is young, education costs are higher in the United States. Another effect of the increased number of young people is the larger market for goods and services. Furthermore, these young people will eventually be contributing to Social Security to help support the elderly. A younger population also indicates a smaller proportion of older people. In 2001, 13 percent of the U.S. population was over age 65, compared with 18 percent in Japan, and 15 percent in Europe.

The average age of the American population is, however, older than it once was, and projections indicate the percentage of the population over 65 will continue to increase through the first quarter of the 21st century. In the first census of the United States, taken in 1790, about half of the white male population was under the age of 16. This extremely youthful society was a result of the high birthrate and the relatively low life expectancy that prevailed in the 18th century. No figures exist on the elderly at that time, but the percentage was undoubtedly quite small. By 1890 the proportion of the population under age 15 had fallen to 35.5 percent, in large part because of the declining birthrate. Only 3.9 percent of the population was over age 65. The median age of the population—that is, the age at which half the people are older and half are younger—had risen to 22. By 2001, the proportion of the population under age 15 had fallen to 21.1 percent, while 12.6 percent of the population was over age 65. The median age in 1990 was 32.8, and according to estimates it had increased to 35.9 by 2001.

The rapid increase in the median age between 1990 and 2000 was the result of the aging of the baby-boom generation—people who were reaching their 30s, 40s, and 50s. The percentage of those under age 5 increased by 4.5 percent during these years, while the percentage of the population between 50 and 54 increased by some 55 percent. The numbers of those between 65 and 69 years of age actually decreased between 1990 and 2000, a reflection of the decline in birthrate during the 1930s depression.

Age differences also vary by ethnicity and race. The median age in 2000 for the non-Hispanic white population was 37.7, for non-Hispanic blacks 30.2, for Native Americans 28.0, for Asian and Pacific Islanders 27.5, and for Hispanics was 24.6. These differences stem in large measure from differences in birthrates.

Economists look carefully at the proportions of the population under age 15 and over 65. They assume people in these age groups do not hold paying jobs and therefore depend for support on those of working age (between 16 and 64). The proportion of dependents (meaning nonworking people) to working-age people suggests the productive capacity of the economy and the social expenses of providing for the nonworking population. In 1790 the proportion of workers to dependents was roughly 50-50. Supporting so many dependents absorbed substantial proportions of social resources and thus slowed economic growth. By 1890 the proportion in America had shifted in favor of those of working age, and about 40 dependents existed for every 60 workers. In the late 1990s there were 35 dependents for every 65 workers.

At the beginning of the 21st century, the proportion of elderly people in the population was increasing, meaning that there were fewer workers per dependent over 65. With the oldest members of the baby-boom generation expected to reach retirement age in 2011, the ratio of workers to dependents will drop even further. This aging of the population poses complex questions, such as how to provide funding for the Social Security system, whether to make medical insurance more widely available, determining who should pay for long-term care of the elderly, and questioning the meaning of retirement. It is unclear how old age will be experienced in the future. The division of social resources between the youngest and the oldest Americans, for example, between schools and retirement communities, has become a matter of considerable debate.

Within the United States, the age structure of the population varies from one region to another and is influenced by people moving into and out of particular regions as well as by the residential choices immigrants make. People tend to move between the ages of 15 and 25 as they attend schools and universities away from home, find apprenticeships and training programs, and seek job opportunities. After age 35 many people have established careers, started families, and made friends and connections, and are less likely to move.

The states that attract newcomers, such as Alaska, Colorado, Georgia, and Texas, tend to have the highest proportion of young people and the smallest proportion of older people. Job opportunity is the most frequent reason for moving, although recreational and environmental considerations are also important. Those who move also consider the available housing stock and the cost of living. Of all the states, Utah had the largest portion of young people at the beginning of the 21st century, largely because of high birthrates among its predominantly Mormon population. The states that experience more people leaving than arriving tend to have fewer young people and more older ones. Such states include Rhode Island, Pennsylvania, West Virginia, and North Dakota. Similarly, many northeastern cities have large elderly populations, while suburbs in the Southeast and Southwest have large populations of younger people. Florida is an exception to these trends, because it attracts many retirees as well as younger Cubans, Haitians, and other immigrants.