Today, the U.S. Census Bureau released population estimates by age, sex, race and Hispanic origin for the nation, states and counties. These data enable us to learn about the U.S. population, including its age structure. Age structure is often displayed using a population pyramid. You can learn about the makeup of the U.S. population as a whole by looking at its population pyramid, below.

An examination of this population pyramid reveals peaks and valleys. Why do the age groups have different size populations? Let’s examine the baby boom generation (50- to 69-year-old population). During the baby boom, the U.S. population rapidly grew because of high fertility rates following World War II. This population surge is reflected in the U.S. population pyramid as an outward bump in those baby boomer age groups. As the baby boomers grew up, many had kids of their own. The children of these baby boomers, frequently called the echo boomers or millennials, can be seen as a similar bulge in the 15- to 34-year-old population. Even with lower fertility among baby boomers compared to their parents’ generation, the birth of the millennials still represented a mini population boom, simply because there were so many potential boomer parents.

Looking at the U.S. population pyramid, we also see how noticeably larger the older female population (age 80 and over) is when compared to the male population at the same ages. This size differential stems from the fact that, generally, women live longer than men do. As a result, older women tend to outnumber older men. Women’s higher share of the older population is one of the more consistent features in almost all population pyramids, regardless of region or level of geography.

We can see more detail when we study population pyramids for smaller geographies, such as states, metropolitan areas or counties. Let’s take a tour of a few interesting examples. We start with West Virginia, which experienced natural decrease (more deaths than births) between 2014 and 2015.

West Virginia’s aging population is visible in the shape of the population pyramid above. Since some of the age groups near the top are wider than those at the bottom, we can tell that there are more older people than younger people. Similar to what was seen in the U.S. population pyramid, the baby boom generation is visible and distinct from the rest of the population. Meanwhile, the number of people of childbearing age (those roughly between the ages of 15 and 49) is comparatively smaller. This shape often leads to natural decrease because of deaths to the larger older population and lack of births from a smaller young population. Many rural areas have long-standing trends of natural decrease and a loss of people through migration, making age structures like this one common. In West Virginia, there have been seven consecutive years of natural decrease and three consecutive years of net migration loss.

On the other end of the population pyramid spectrum is the Salt Lake City metropolitan area in Utah. Here we see growth from both positive net migration and natural increase (where more people are being born than are dying). Looking at Salt Lake City’s population pyramid, you can see that there are relatively more people that are of childbearing age, especially when compared to the older population. Additionally, a long history of natural increase is evident in Salt Lake City’s age structure, since it gradually gets wider toward the bottom. As high fertility levels [PDF 2.9 MB] and net migration gains persisted, the base of Salt Lake City’s population pyramid widened over the years, resulting in the shape it has now.

Births and deaths aren’t the only components that influence the shape of a population’s age structure. Net migration (both domestic and/or international) can play a major role as well. Ouray County, Colo., for example, is defined by the Economic Research Service at the U.S. Department of Agriculture as a recreation county. Roughly speaking, being a recreation county means that the number of people who make their livings through recreational activities and the number of seasonal housing units are both relatively high. Recreation counties generally have similar age structures to one another. In Ouray County’s case, you can see the proportionately large share in the 50- to 74-year-old age groups. Like many recreation counties, in Ouray County, the primary driver of population growth is net domestic migration. Much of that growth was in the 65-and-older population, which implies that a substantial number of people are retiring in Ouray County.

We can contrast the “older” age structure in Ouray County, Colo., with Centre County, Pa., which contains a large university. As can be seen in many college counties, Centre County has a noticeable spike in the 15- to 24-year-old population. Centre County gained residents between 2014 and 2015 from both natural increase and net migration, but — like Ouray County — net migration was the primary driver of its population increase. In the last year, Centre County experienced a decrease in the population under age 25 and an increase in the 25-and-older population. Despite these changes, the college-age population is a distinctive feature in Centre County’s population pyramid.

Areas with large military installations can also have unusual age structures. Take, Christian County, Ky., for example. Christian County is the location of part of a large military installation. We see the impact on the population pyramid as a disproportionately large male 20- to 29-year-old population relative to all other age and sex groups. In Christian County’s case, the resident population decreased slightly in the last year due primarily to a net loss of domestic migrants. Specifically, while the 18-to 24-year old and 65-and-older age groups increased slightly in the last year, this increase was offset by the loss of people ages 25 to 64 and children under the age of 18. As a side note, a similar kind of age structure to Christian County’s is seen often, but in a very different kind of place. Aside from the bulge of young children (less than 10), the age structure seen in Christian County would resemble the shape of counties with proportionally large incarcerated population.

As you can see in Christian County, and the rest of the U.S., age structure tells us a lot about an area’s population and how it changes over time.