WASHINGTON — Children now make up less of America’s population than ever before, even with a boost from immigrant families, according to census figures. Currently, the share of children in the U.S. is 24 percent, falling below the previous low of 26 percent of 1990.

And when this generation grows up, it will become a shrinking work force that will have to support the nation’s expanding elderly population — even as the government strains to cut spending for health care, pensions and much else.

The latest 2010 census data show that children of immigrants make up one in four people younger than 18, and are now the fastest-growing segment of the nation’s youth, an indication that both legal and illegal immigrants as well as minority births are lifting the nation’s population.

Currently, the share of children in the U.S. is 24 percent, falling below the previous low of 26 percent of 1990. The share is projected to slip further, to 23 percent by 2050, even as the percentage of people 65 and older is expected to jump from 13 percent to 19 percent because of the aging of baby boomers and beyond.

In 1900, the share of children reached as high as 40 percent, […]