The information from the Census Bureau highlights the impact of recent waves of young Mexican immigrants and their children, who are helping to slow the aging of the population in many parts of the United States. It is reinforced by fresh data released Thursday that show a median-age jump of 2.5 years or more over the last decade for states including New Hampshire, Wisconsin, Ohio and Connecticut where there are fewer immigrants.
"The census numbers show that we are really splitting apart between regions that are gaining younger people and families with children and those that are getting older and where traditional families are becoming scarce," said William H. Frey, a demographer at Brookings Institution.
He cited several factors leading to the growing divide, including aging of the nation's 78 million mostly white baby boomers, who are now between the ages of 46 and 65 and past their prime childbearing years. Their demographic impact is being keenly felt in the Northeast and Midwest, especially in the current economic slump, as many young adults and immigrants seek better job opportunities in the Sun Belt.
"The age divide has a race-ethnic dimension, and it will certainly play a role in politics that pits benefits for seniors with those for younger adults and their children," Frey said.
The western U.S. region — which includes states with sizable Hispanic populations such as California, Arizona, New Mexico and Colorado — had the nation's lowest median age last year at roughly 35.1, compared with 39 in the Northeast and 37.5 in the Midwest, according to census estimates.
That age difference of 3.9 years with the Northeast and 2.4 with the Midwest is nearly double the levels in 1990, when the oldest boomers were beginning to move out of their prime childbearing years.
The South, the second-youngest region, with a median age of 36.4, had an age gap of 2.6 years with the Northeast and 1.1 with the Midwest. Before 1990, when immigrants began flowing in larger numbers across the U.S.-Mexican border, the median ages of the South, Midwest and West were all roughly equal at 29.
Leonard Steinhorn, an American University professor and author of "The Greater Generation: In Defense of the Baby Boom Legacy," sees the potential for sharpened generational politics in aging parts of the country such as Ohio, Pennsylvania, Wisconsin and Michigan where people 45 and older make up the majority of voters.
"There's a reason politicians don't run around talking that much these days about the cost of college loans or education, which affect younger people," he said. "The problem is when you have limited resources, you end up with the potential to pit one generation against the next."
Citing the high representation of older voters in 2010 who disproportionately voted Republican amid debate over a health overhaul, Steinhorn added, "If seniors vote more in elections and throw some people out, politicians may respond more to the concerns of seniors and take for granted youths."
The census figures come amid debate over hotly contested issues, including immigration and how to address the rising cost of Medicare, that are certain to resonate in differing ways by region in 2012.
In a bid to cut the federal debt, House Republicans initially pushed a plan that would replace Medicare with a government payment individuals would use to buy private insurance. Although the plan would only affect those under 55, Republicans have pulled back after meeting stiff protests from Democrats and older voters in bellwether states such as Florida, New Hampshire and Wisconsin.
House Speaker John Boehner, R-Ohio, and other Republicans have indicated that without an adequate budget agreement on curbing the costs of Medicare and other entitlements, bigger cuts would be required elsewhere. Previous GOP proposals have targeted discretionary domestic spending programs such as education.
Meanwhile, President Barack Obama is seeking to make immigration overhaul a wedge issue against Republicans, telling voters along the border in El Paso, Texas, this week to push Congress to pass legislation providing a pathway to citizenship for 11 million illegal immigrants. His move comes amid rapid Mexican population growth mostly in the South and West that could help tip swing states but has also prompted Republican-controlled legislatures in Arizona and Utah to pass stringent laws seeking to limit illegal immigration.
Mexican immigrants, who are younger and more likely to have children, tend to lean Democratic if they vote.
In all, 12 of the 14 states plus the District of Columbia with median ages of 36 or younger are located in the South and West, according to census figures. They are Alaska, Arizona, California, Colorado, Georgia, Idaho, Louisiana, Mississippi, Nevada, Oklahoma, Texas and Utah.
By contrast, 13 of the 20 states with median ages of 38 or higher fell in the Midwest and Northeast. They are Connecticut, Iowa, Maine, Massachusetts, Michigan, New Hampshire, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, Vermont and Wisconsin.
Nationally, the median age is about 36.8, up from 35.3 in 2000.
The figures are based on the Census Bureau's 2009 American Community Survey, which samples 3 million U.S. households, as well as 2010 census data on age, household relationships and racial and ethnic groups for the 24 states and the District of Columbia that have been released so far this month.
Among the findings:
—Big cities posted jumps in median age as young families moved to the suburbs or for jobs elsewhere: Detroit, increasing nearly 4 years to 34.8; Cleveland, 2.7 years to 35.7; and Los Angeles, 2.5 years to 34.1. On the other hand, Boston and the District of Columbia saw drops in median age, to 30.8 and 33.8 years, respectively, as young adults moved in to attend colleges or seek jobs in burgeoning biotech industries in the metro areas.
—Mexicans accounted for more than half of the total Hispanic gains in roughly 38 states over the last decade, with the biggest shares found in states near or along the U.S.-Mexico border, including New Mexico, Texas, Arizona, Nevada and California.
—Traditional married couples with children were most likely found in Utah, Idaho, Texas and California, making up nearly 1 in 4 households there. Non-family households typically composed of singles ages 65 and older lived mostly in Midwestern or Northeastern states such as North Dakota, Massachusetts, Vermont, Rhode Island and Maine.
—Nevada, Texas and Wyoming, which had strong economies that helped attract young workers or immigrants for at least portions of the last decade, were among the states with some of the smallest increases in median age of roughly 1 year. Texas also gained nearly 1 million children, representing about half the U.S. total gain in children; the ones in Texas were almost all Hispanic.