For the first time, half (49.9%) of American children under the age of five are of a racial or ethnic minority, according to the US Census Bureau.
The agency projected that in the US, whites would become a minority in the under-five age group this year or next.
The 2012 data also found that for the first time in more than a century, there were more deaths than births among white Americans.
The "natural decrease" occurred several years earlier than forecast, it said.
The government figures also project that in five years, minorities will make up more than half of children under 18.
The Census Bureau expects further decline among the white population as aging baby-boomers die.
The latest findings come a year after the census reported that whites had fallen to a minority among babies in the US.
Minority populations are growing faster than the white community due to high birth rates, especially in the Hispanic community, as well as immigration.
But the white American population is still seeing small increases because of immigration from Europe, the data shows.
The Census Bureau said 63% of Americans were white and non-Hispanic.
The fastest-growing minority groups were Asians, whose population expanded by 2.9% in 2012, and Hispanics, by 2.2%.