For the first time, America's racial and ethnic minorities make up about half of the under-5 age group, the US government has said.
The historic shift, announced on Thursday, shows how young people are at the forefront of sweeping changes by race and class.
The new census estimates, a snapshot of the US population as of July 2012, come a year after the Census Bureau reported that whites had fallen to a minority among babies.
The population younger than 5 stood at 49.9 percent minority in 2012.
"More so than ever, we need to recognise the importance of young minorities for the growth and vitality of our labour force and economy," said William H. Frey, a demographer at the Brookings Institution who analysed the census data.
Fuelled by immigration and high rates of birth, particularly among Hispanics, racial and ethnic minorities are now growing more rapidly in numbers than whites.
Based on current rates of growth, whites in the under-5 group are expected to tip to a minority this year or next, said Thomas Mesenbourg, the Census Bureau's acting director.
The government also projects that in five years, minorities will make up more than half of children under 18.
Not long after that, the total US white population will begin a decline in absolute numbers, due to aging baby boomers.
The nation's demographic changes are already stirring discussion as to whether some civil rights-era programmes, such as affirmative action in college admissions, should be retooled to focus more on income rather than race and ethnicity.
The Supreme Court will rule on the issue this month.
The gap between rich and poor in the US has now stretched to its widest since 1970, making opportunities to reach the middle class increasingly difficult.
Longer-term changes in family structure, such as a decline in marriage, have led to a rise in single-mother households across all racial groups, with the fastest growth now occurring among whites.
More than 40 percent of newborns are now born out of wedlock, in families more likely to be low income.
As a whole, the nonwhite population increased by 1.9 percent to 116 million, or 37 percent of the U.S. The fastest percentage growth is among multiracial Americans, followed by Asians and Hispanics. Non-Hispanic whites make up 63 percent of the U.S.; Hispanics, 17 percent; blacks, 12.3 percent; Asians, 5 percent; and multiracial Americans, 2.4 percent.
Among the under-5 age group, 22 percent live in poverty. Black toddlers were most likely to be poor, at 41 percent, followed by Hispanics at 32 percent and whites at 13 percent. Asian toddlers had a poverty rate of 11 percent.