The census showed record growth in the Greater Washington area between 2010 and 2020, from 5.6 million to 6.4 million people, according to a new report by Urban Institute fellow Peter Tatian.

Most of this population growth came from the addition of new adults – the region added 639,000 adults during this decade, an increase of 15%. Meanwhile, the population of children under 18 increased by only 7%, or 96,700. For each additional child, the region added about five adults.

The district, which has seen similar trends to the region, has a larger share of adults in its population than any state, according to census data. 83.4% of DC residents were over the age of 18 in 2020, compared to 77.9% of the United States as a whole.

But the Maryland counties in the region were the ones whose adult population grew the most, adding 12 adults for every child. Maryland’s population growth has been driven by increasing Hispanic/Latino, Black, Asian, and Pacific Islander populations.

The share of adults in the U.S. population is also growing, according to the Census Bureau, a combination of aging baby boomers and declining fertility since 2007. And unlike the Washington area, the total number of children in United States in general decreased between 2010 and 2020 by 1.4%.

Although the district has a larger share of adults than any state, those adults are young — in fact, in 2019 only one state, Utah, had a younger median age. The median DC resident in 2020 was 34 years old.