The U.S. under-18 population fell between 2010 and 2011, the first time in at least two decades that the country has seen its minor population decline, according to demographers and new Census data.

The U.S. under-18 population was 73,934,272 in July 2011, a decline of 247,000 or 0.3% from July of 2010, according to an analysis of Census data by William H. Frey, a demographer atThe Brookings Institution. The child population is still up 2.3% from 2000, largely because of gains made in the early-decade boom years.

The child population is falling because fewer immigrant children are coming across U.S. borders, and because fewer children are being born. Meantime, the so-called millennial generation is moving into adulthood. With fertility rates down, Mr. Frey says “it doesn’t look like a youth boom will reverberate anytime soon.”

The U.S. minor population fell in the 1970s as well, as baby boomers moved into adulthood and women entered the labor force en masse, delaying families in the process. A large drop in fertility was also behind a decline in minors between 1920 and 1930.

States with the biggest drop in children tended to be concentrated in the aging Rust Belt and New England. Every New England state saw its under 18 population fall 1% or greater from April 2010 to July 2011 (state Estimates are over a different time period than the national tally). Michigan and Pennsylvania were also big losers. Also, while the drops were small, states including Arizona and Nevada saw their minor populations fall after huge gains earlier in the decade.