WASHINGTON (AP) - For the first time, America's racial and ethnic minorities make up about half of the under-five age group, the US government said on Thursday.

It is a historic shift that shows how young people are at the forefront of sweeping changes by race and class.

The new census estimates, a snapshot of the U.S. population as of July 2012, come a year after the Census Bureau reported that whites had fallen to a minority among babies. Fueled 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 U.S. 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 programs, 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 U.S. 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 per cent of newborns are now born out of wedlock, in families more likely to be low income.

The latest census numbers show:

-The population younger than five stood at 49.9 per cent minority in 2012.

-For the first time in more than a century, the number of deaths now exceeds births among white Americans. This "natural decrease" occurred several years before the government's original projection. For now, the white population is still increasing slightly, due to immigration from Europe.

-As a whole, the non-white population increased by 1.9 per cent to 116 million, or 37 per cent of the US. The fastest percentage growth is among multiracial Americans, followed by Asians and Hispanics. Non-Hispanic whites make up 63 per cent of the US; Hispanics, 17 per cent; blacks, 12.3 per cent; Asians, 5 per cent; and multiracial Americans, 2.4 per cent.

-Among the under-five age group, 22 per cent live in poverty. Black toddlers were most likely to be poor, at 41 per cent, followed by Hispanics at 32 per cent and whites at 13 per cent. Asian toddlers had a poverty rate of 11 per cent.

"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 Frey, a demographer at the Brookings Institution who analysed the census data.

The gaps in achievement tend to emerge early in childhood, and disparities are especially evident in SAT admission scores.

College Board data show that average scores spread as wide as 130-140 points in each of the reading, math and writing sections for a student with family income of less than US$20,000 (S$24,771), compared with a student with family income exceeding US$200,000.

About 40 per cent of whites age 25-29 graduate from college, compared with 15 per cent for Latinos and 23 per cent for blacks.