Driven by immigrants and young people moving to the South and West and older Americans who stay put elsewhere, the age gap between regions in the U.S. has grown to its widest level in decades, sharpening the divides on hot-button issues such as immigration and Medicare.
The new numbers from the Census Bureau highlight 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.
"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.
"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," Mr. 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 years, 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 is the second-youngest region with a median age of 36.4. Before 1990, when immigrants began flowing in larger numbers across the U.S.-Mexico 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 a 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."
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:
c 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.
c 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.