By comparison, the younger population under age 18 numbered 73.1 million, or 22.1% of the U.S. population in 2020, a 1.4% decrease from 74.2 million in 2010. The slow decline of the younger population is in part due to a general decrease in fertility, ongoing since 2007.

Source: Adult Population Grew Faster Than Total Population From 2010 to 2020

Actually, the fertility rate has been in decline for a much longer time.

Coldstreams Skeptic