Median Age Doesn’t Tell the Whole Story

The nation’s median age was 38.2 in 2018, up from 37.2 in 2010, but new county population characteristics released by the U.S. Census Bureau today show that age is more than a number.

Two or more counties can have the same median age — the point when half the population is older and half younger — yet have age profiles of their populations that are completely different.

Some may have relatively large shares of young adults and not many children or older people while other counties can have large proportions of children and 35- to 59-year-olds (i.e., their parents).  Despite these compositional differences, these counties can have roughly the same median age.

To illustrate this, let’s look at three counties that have roughly the same median age as the nation and as each other.

California’s San Francisco County — which has the same boundary as the city of San Francisco — had a median age of 38.2 in 2018, a slight decrease from 38.5 in 2010.

San Francisco County is home to many 25- to 39-year-olds who collectively make up almost a third (32.5%) of the total county population. The “population pyramid” below shows how the size of the county’s age groups, broken down by sex, is distributed.

Now compare San Francisco County with Carver County, Minnesota, which is a part of the Minneapolis-St. Paul-Bloomington, MN-WI metropolitan area.

In 2018, Carver County’s median age was 38.1, up from 36.3 in 2010. The age structure in Carver County has two distinct groups: those under the age of 20 and those ages 35 to 59 — basically children and the parents of those children.

Carver County has about the same median age as San Francisco County, but its age structure is almost the inverse. One has a large population of children and 35- to 59-year-olds (Carver) while the other has a lot of 25- to 39-year-olds (San Francisco).

Some counties have no one age group that sticks out.

For example, Greenville County in South Carolina, part of the Greenville-Anderson-Mauldin, SC metropolitan area, had a median age of 38.2, up from 37.2 in 2010.

Greenville County’s median ages in 2010 and 2018 were the same as the nation’s, and its age structure is very similar to the nation’s as well. The population is more evenly distributed across most age groups, unlike the distinct age groups that dominate in San Francisco County and Carver County.

Median age is useful in summarizing whether a population is aging, but it’s important to remember that there is more to the age structure of the population than the snapshot that median age alone can provide.

Luke Rogers is chief of the Census Bureau’s Population Estimates Branch in the Population Division.