The number of young children in the U.S. fell 9% in the decade ending 2020 as the portion of Americans 65 and over ballooned more than 38%, according to U.S. Census data released Thursday.

The dual patterns lifted America’s median age from 37 to nearly 39, the bureau said, a rise of more than a decade over a half-century. In 1970, half of the U.S. population was barely 28 years old or younger.

Experts say the decline in small children is a lingering effect of the Great Recession of 2007-2009, with many women choosing to wait until later in life to have babies. Birth rates have not recovered since.