the older half. Although the median age, viewed over time, is a useful summary indicator of population aging, it does not capture many important compositional changes so crucial for public policy decisions. Population aging usually involves a decreasing proportion of young people and, correspondingly, an increasing proportion of the older ones. For many issues, it is helpful to separate younger and older groups and present estimates of their population size and change. But, first, we discuss the relationship of immigration and median age.
Population aging is not a new phenomenon in the United States. In the 1800s, the median age of the U.S. population was under 20, a reflection of high fertility levels that produced a population with a large number of children. The median age has been steadily rising for more than a century, reaching the historically unprecedented level of over 34 in 1995.
As the results below indicate, the median age of the population will continue to rise, changing as much in the next 55 years as in the previous 55 years. This rise is a consequence of continued low levels of fertility and the aging of the numerically large baby-boom cohorts. Over the next half-century, the impact of the baby-boom cohorts of the 1950s will be clear. Under all the net immigration assumptions, the future population will continue to age (Figure 3.4). Under the medium assumption, the median age will rise to 38.5 years in 2030 and then level off at about 38 years. Under the low net immigration assumption, the population