Preliminary census estimates also show the share of African-American households headed by women -- mostly single mothers -- now exceeds African-American households with married couples, a sign of declining U.S. marriages overall but also of continuing challenges for black youths without involved fathers.
The findings, based on the latest government data, offer a preview of final 2010 census results being released this summer that provide detailed breakdowns by age, race and householder relationships.
Demographers say the numbers provide the clearest confirmation yet of a changing social order, one in which racial and ethnic minorities will become the U.S. majority by midcentury.
"We're moving toward an acknowledgment that we're living in a different world than the 1950s, where married or two-parent heterosexual couples are now no longer the norm for a lot of kids, especially kids of color," said Laura Speer, coordinator of the Kids Count project for the Baltimore-based Annie E. Casey Foundation.
"It's clear the younger generation is very demographically different from the elderly, something to keep in mind as politics plays out on how programs for the elderly get supported," she said. "It's critical that children are able to grow to compete internationally and keep state economies rolling."
Currently, non-Hispanic whites make up just under half of all children 3 years old, which is the youngest age group shown in the Census Bureau's October 2009 annual survey, its most recent. In 1990, more than 60 percent of children in that age group were white.
William H. Frey, a demographer at the Brookings Institution who analyzed the data, said figures in the 2009 survey can sometimes be inexact compared with the 2010 census, which queries the entire nation. But he said when factoring in the 2010 data released so far, minorities outnumber whites among babies under age 2.
The preliminary figures are based on an analysis of the Current Population Survey as well as the 2009 American Community Survey, which sampled 3 million U.S. households to determine that whites made up 51 percent of babies younger than 2. After taking into account a larger-than-expected jump in the minority child population in the 2010 census, the share of white babies falls below 50 percent.
Twelve states and the District of Columbia now have white populations below 50 percent among children under age 5 -- Hawaii, California, New Mexico, Texas, Arizona, Nevada, Florida, Maryland, Georgia, New Jersey, New York and Mississippi. That's up from six states and the District of Columbia in 2000.
At current growth rates, seven more states could flip to "minority-majority" status among small children in the next decade: Illinois, North Carolina, Virginia, Colorado, Connecticut, South Carolina and Delaware.
By contrast, whites make up the vast majority of older Americans -- 80 percent of seniors 65 and older and roughly 73 percent of people ages 45-64. Many states with high percentages of white seniors also have particularly large shares of minority children, including Arizona, Nevada, California, Texas and Florida.
"The recent emergence of this cultural generation gap in states with fast growth of young Hispanics has spurred heated discussions of immigration and the use of government services," Frey said. "But the new census, which will show a minority majority of our youngest Americans, makes plain that our future labor force is absolutely dependent on our ability to integrate and educate a new diverse child population."
Kenneth Johnson, a sociology professor and senior demographer at the University of New Hampshire, noted that much of the race change is being driven by increases in younger Hispanic women having more children than do white women, who have lower birth rates and as a group are moving beyond their prime childbearing years.
Because minority births are driving the rapid changes in the population, "any institution that touches or is impacted by children will be the first to feel the impact," Johnson said, citing as an example child and maternal health care that will have to be attentive to minorities' needs.
The numbers come amid public debate over hotly contested federal and state issues, from immigration and gay marriage to the rising cost of government benefits such as Medicare and Medicaid, that are resonating in different ways by region and demographics.
Alabama became the latest state this month to pass a wide-ranging anti-immigration law, which in part requires schools to report students' immigration status to state authorities. That follows tough immigration measures passed in similarly Republican-leaning states such as Georgia, Arizona and South Carolina.
But governors in Massachusetts, New York and Illinois, which long have been home to numerous immigrants, have opted out of the federal Secure Communities program that aims to deport dangerous criminals, saying it has made illegal immigrants afraid of reporting crimes to police. California may soon opt out as well.
States also are divided by region over old-age benefits and gay marriage, which is legal in five states and the District of Columbia.
Among African-Americans, U.S. households headed by women -- mostly single mothers but also adult women living with siblings or elderly parents -- represented roughly 30 percent of all African-American households, compared with the 28 percent share of married-couple African-American households. It was the first time the number of female-headed households surpassed those of married couples among any race group, according to census records reviewed by Frey dating back to 1950.
While the number of black single mothers has been gradually declining, overall marriages among blacks are decreasing faster. That reflects a broader U.S. trend of declining marriage rates as well as increases in non-family households made up of people living alone, or with unmarried partners or other non-relatives.
Female-headed households make up a 19 percent share among Hispanics and 9 percent each for whites and Asians.
--Multigenerational households composed of families with grandparents, parents and children were most common among Hispanics, particularly in California, Maryland, Illinois, Nevada and Texas, all states where they represented nearly 1 in 10 Latino households.
--Roughly 581,000, or a half percent, of U.S. households are composed of same-sex unmarried couples, representing nearly 1 in 10 households with unmarried partners. Unmarried gay couples made up the biggest shares in states in the Northeast and West, led by the District of Columbia, Oregon, Delaware, Massachusetts, Maine and Vermont. The largest numbers were in California and New York, which is now considering a gay marriage law.
--Minorities comprise a majority of renters in 10 states, plus the District of Columbia -- Hawaii, Texas, California, Georgia, Maryland, New Mexico, Mississippi, New Jersey, Louisiana and New York.
Tony Perkins, president of the Washington, D.C.-based Family Research Council, a conservative interest group, emphasized the economic impact of the decline of traditional families, noting that single-parent families are often the most dependent on government assistance.
"The decline of the traditional family will have to correct itself if we are to continue as a society," Perkins said, citing a responsibility of individuals and churches. "We don't need another dose of big government, but a new Hippocratic oath of `do no harm' that doesn't interfere with family formation or seek to redefine family."