In the past decade, the U.S. population of those 65 or older swelled by more than a third as the large generation of baby boomers began to hit retirement age, according to data the government released Thursday.

Between 2010 and 2020, the population of people 65 and older rose 38.6 percent, from 40.3 million people in 2010 to 55.8 million in 2020, according to newly released 2020 Census data that provides more details on the population, by age, sex, race, ethnicity, families and households, and housing occupancy. The country's median age is now 38.8, up from 37.2 in 2010.

Baby boomers, born between 1946 and 1964, began to turn 65 in 2011. Demographers and policymakers have long warned that as they retire, and as fertility and immigration rates decline, there will be fewer people in the labor force to support them.

The under-18 population dipped by 1.4 percent between 2010 and 2020, from 74.2 million in 2010 to 73.1 in 2020, the data shows. The largest decline was among under-5 children, a group that dropped by 8.9 percent or 1.8 million, reflecting a decline in births and the birthrate since 2015.

The trends put pressure on the economic balance between people in and out of the labor force, in which the working-age population supports those too old, or too young, to work. A figure known as the total dependency ratio provides a rough idea of economic dependency by dividing children and those 65 and older by the working-age population (18 to 64). Between 2010 and 2020, the total dependency ratio rose from 58.9 to 63.6, and the old-age dependency ratio rose from 20.7 to 27.5.

The old-age ratio is of the biggest concern, said William Frey, a senior demographer at the Brookings Institution.

"It essentially tells how much the retirement-age population is dependent on the working age population, and it continues to rise rapidly due to the aging of the baby boom," he said. "While the current level has now risen to near 30, we could be headed toward the mid-30s, which would mean more than a third of seniors would be dependent on a slower-growing working-age population. U.S. projections suggest this could rise higher, especially in the absence of healthy immigration."

The oldest state in the country was Maine, with a median age of 45.1; the youngest was Utah, with a median age of 31.3. Virginia and Maryland hovered around the national median, at 38.7 and 38.8, respectively. The median age in the District of Columbia was 33.9, and in the Washington metro area, it was 37.2.

Homeownership rates dipped to their lowest level since 1970, the data showed. Of the 126.8 million occupied housing units in 2020, 80.1 million, or 63.1 percent, were owner-occupied, a decrease of 2 percentage points since 2010.

The country also became more diverse overall, with the under-18 group going from 53.5 to 47.3 percent non-Hispanic Whites between 2010 and 2020. The White share also declined in the 65-and-over population, from 80 to 74.8 percent, according to an analysis of the data conducted by Frey.

But in D.C., the White population for both age groups rose over the decade, going from 30.7 to 33.6 percent of the 65-and-older population and from 17.4 to 23.6 percent for the under-18 group. Both age groups also saw a decline in Black population share in D.C., from 61.7 to 55.2 percent for those 65 and older and from 65.3 to 51.2 percent for those under 18.

The Washington metropolitan area grew more diverse over the decade for both groups, going from White shares of 41.4 to 33.7 percent of the under-18 population and from 63.3 to 56.8 percent of those 65 and older.

But the District's two-decade growth in White population reversed itself during the first year of the pandemic - between July 2020 and July 2021 - when many people left the city, according to Census Bureau data released a year after the decennial census came out.

For four years leading up to the pandemic, the city had added non-Hispanic White residents at a rate of about 4,000 to 5,000 each year. But in the first year of the pandemic, it lost 10,285 White people, according to the bureau's annual population estimates released last year. White people made up half of the District's decline even though they make up only 37 percent of the city's population.

The city's Black population, which had already been declining, dropped more between 2020 and 2021. The city lost 6,689 non-Hispanic Black residents that year, over 3 1/2 times the number it had lost the previous year. And the majority-Black Prince George's County, which had also been losing Black residents, lost 8,552 during the first year of the pandemic, around three times as many as the previous year.

However, among 142 counties with populations exceeding 500,000, the District is doing relatively well. It ranks fourth for growth of population of people ages 25 to 44, third in share of population ages 25 to 44, and second in share of population ages 18 to 64, and it has the ninth-lowest median age, according to Frey's analysis.

"That, along with its White growth in the 2010s, made it one of the nation's strongest youthful magnets," Frey said. "While the pandemic slowed this, recent estimates suggest it is coming back."