longer (e.g., longevity), and the proportion of children in the population decreases because of lower birthrates. Both of these trends took place throughout the 20th century and have continued into the early 21st century, but the drop in the numbers of children being born is a more significant factor for population aging than is people living longer. In 1900, the United States had a relatively young population: The percentage of children and teenagers in the population was 40%. According to U.S. Census Bureau data, by 2017, the proportion of youth had dropped to 24%, an all-time low. By contrast, those ages 65 and older increased from 4% in 1900 to 16% in 2017, with larger increases still to come (Kaiser Family Foundation, 2017b). During the next several decades, overall population growth in the United States will be concentrated among middle-aged and older Americans.
The United States is not the only country undergoing population aging (Bosworth & Burtless, 1998; Cherlin, 2010; National Institute on Aging, 2007). For example, average life expectancy at birth in Japan is currently 84 years (World Population Review, 2020), the highest in the world, and the proportion of the population ages 65 and older there is 27% (World Bank, 2019). In Germany, Italy, and Japan, the population is aging because of low birthrates as well. Think of the state of Florida today as a model for population aging: a population in which nearly one in five people is already over the age of 65. We can ask: How long will it take different nations to reach the condition of “Florida-ization”? The answer is that Italy already looked like Florida by 2003, Japan by 2005, and Germany by 2006. France and Great Britain resembled Florida in 2016, whereas the United States in general will not reach “Florida-ization” until 2023.
Exhibit P.1 Life Expectancy at Birth, by Sex, Race, and Hispanic Origin: United States, 1980–2014
Source: Figure 18 in Centers for Disease Control and Prevention (2015); CDC/NCHS, National Vital Statistics System.
Note: Life expectancy data by Hispanic origin were available starting in 2006 and were corrected to address racial and ethnic misclassification.
Population aging also shows up as an increase in the median age for the entire population, that is, the age at which half the population is older and half the population is younger. The median age of the U.S. population in 1820 was only 17 years; by 1900 it rose to 23 and by 2018 to 38. It is estimated that the median age of the American population by 2030 will be 42 years. This shift is a measure of the dramatic impact of population aging.
It is clear, then, that populations age for reasons different than individuals do, and the reasons have to do with large-scale demographic trends. In the first place, population aging occurs because birthrates go down. With a smaller proportion of children in the population, the average age of the population goes up. Population aging can also come about because of increases in life expectancy—people living longer on average. Finally, the process of population aging can be influenced for a time because of the characteristics of birth cohorts. A cohort is a group of people born during a particular time who thereby experience common life events during the same historical period. For example, the cohort born during the Great Depression of the 1930s was relatively small and thus has had minimal impact on the average age of the population. By contrast, the baby boomers born after World War II are a large cohort. Because of this cohort’s size, the middle-aged baby boomers are dramatically hastening the aging of the U.S. population. Even larger than the baby boomer cohort is the Millennial or Gen Y cohort, the members of which were ages 24 to 39 as of 2020 (Pew Research Center, 2020). How will this large cohort, numbering 72.1 million, impact population aging in the U.S. in future decades?
In summary, then, trends in birthrates, death rates, and the flow of cohorts all contribute to population aging. What complicates matters is that all three trends can be happening simultaneously, as they have been in the United States in recent decades. Casual observers sometimes suggest that the U.S. population is aging mainly because people are living longer. But that observation is not quite accurate because it fails to take into account multiple trends defined by demographic factors of fertility, mortality, and flow of cohorts.
A demographic description tells us what the population looks like, but it does not explain the reason that population trends happen in the first place. We need to ask: Why has this process of population aging occurred? The rising proportion of older people in the population can be explained by demographic transition theory, which points to a connection between population change and the economic process of industrialization. In preindustrial societies, there is a generally stable population because both birthrates and death rates remain high. With industrialization, death rates tend to fall, whereas birthrates remain high for a period, so the total population grows. But at a certain point, at least in advanced industrial societies, birthrates begin to fall back in line with death rates. Eventually, when the rate of fertility is exactly balanced by the rate of mortality, we have a condition of stability known as zero population growth (Chu, 1997). The population is neither growing nor shrinking.
The Western industrial revolution of the 19th century brought improved agricultural production, improved standards of living, and also an increase in population size. Over time, there came a shift in the age structure of the population, known to demographers as the demographic transition. This was a shift away from a population with high fertility and high mortality to one of low fertility and low mortality. That population pattern is what we see today in the United States, Europe, and Japan. The result in all industrialized societies has been population aging: a change in the age distribution of the population.
Most countries in the developing world—in parts of Africa, Asia, and Latin America—still have fertility rates and death rates much higher than those of advanced industrialized countries. For the United States in 1800, as for most developing countries today, that population distribution can be represented as a population pyramid: many births (high fertility) and relatively few people surviving to old age (high mortality). For countries that are approaching zero population growth, that pyramid is replaced by a cylinder: Each cohort becomes approximately the same in size.
As we have seen, the increased number of older people is only part of the cause of population aging. It is important to remember that, overall, population aging has been brought about much more by declines in fertility than by reductions in mortality. The trend toward declining fertility in the United States can be traced back to the early 19th century, and the process of population aging has causes that date back even longer (Olshansky & Carnes, 2002). To complete the demographic picture, we need to point to other factors that influence population size and composition, such as improvements in the chance of survival of people at different ages or the impact of immigration into the United States, largely by younger people. But one conclusion is inescapable: Today’s increased proportion of people ages 65 and older springs from causes that are deeply rooted in American society. Population aging is a long-range trend that will characterize our society into the 21st century, driven largely by changes in immigration and the aging of the baby boomers, as well as the even larger millennial cohort. It is a force we all will cope with for the rest of our lives.
But how is American society coping with population aging? How are the major institutions of society—education, health care, government, the economy, the family—responding to the aging of a large number of individuals? The answer, in simplified terms, is rooted in a basic difference between individual and population aging. As human beings, we are all familiar with the life course process of individual aging. It is therefore not surprising that, as a society, we have devised many policies and practices to take into account changes that predictably occur in the later years, such as planning for retirement, medical interventions for chronic illnesses, and familiar government programs such as Social Security and Medicare.
Whether
