Rural America now accounts for just 16 percent of the nation's population, the lowest ever.

The latest 2010 census numbers hint at an emerging America where, by midcentury, city boundaries become indistinct and rural areas grow ever less relevant. Many communities could shrink to virtual ghost towns as they shutter businesses and close down schools, demographers say.

More metro areas are booming into sprawling megalopolises. Barring fresh investment that could bring jobs, however, large swaths of the Great Plains and Appalachia, along with parts of Arkansas, Mississippi and North Texas, could face significant population declines.

These places posted some of the biggest losses over the past decade as young adults left and the people who stayed got older, moving past childbearing years.

For instance in West Virginia, now with a median age of 41.3, the share of Americans 65 and older is now nearly double that of young adults 18-24 — 16 percent compared to 9 percent, according to census figures released Thursday. In 1970, the shares of the two groups were roughly equal at 12 percent.

"This place ain't dead yet, but it's got about half a foot in the grave," said Bob Frees, 61, of Moundsville, W.Va., which now has a population of just over 9,000. "The big-money jobs are all gone. We used to have the big mills and the rolling plants and stuff like that, and you could walk out of high school when you were 16 or 17 and get a $15-an-hour job."

Demographers put it a bit more formally.

"Some of the most isolated rural areas face a major uphill battle, with a broad area of the country emptying out," said Mark Mather, associate vice president of the Population Reference Bureau, a research group in Washington, D.C. "Many rural areas can't attract workers because there aren't any jobs, and businesses won't relocate there because there aren't enough qualified workers. So they are caught in a downward spiral."

Rural towns are scrambling to attract new residents and stave off heavy funding cuts from financially strapped federal and state governments.

Delta Air Lines recently announced it would end flight service to 24 small airports, several of them in the Great Plains, and the U.S. Postal Service is mulling plans to close thousands of branches in mostly rural areas of the country. The University of Kansas this month opened a new medical school with a class of eight in Salina, a regional hub of nearly 50,000 people, in hopes of supporting nearby rural communities that have no doctors at all.

In North Dakota, colleges are seeking to draw in young adults by charging low tuition and fees. It's part of a broader trend in which many slow-growing rural states are touting recreational scenic landscapes or extending tuition breaks to out-of-state residents who typically are charged more.

Many rural areas, the Great Plains in particular, have been steadily losing population since the 1930s with few signs of the trend slowing in coming decades, according to census figures.

The share of people in rural areas over the past decade fell to 16 percent, passing the previous low of 20 percent in 2000. The rural share is expected to drop further as the U.S. population balloons from 309 million to 400 million by midcentury, leading people to crowd cities and suburbs and fill in the open spaces around them.

In 1910, the population share of rural America was 72 percent. Such areas remained home to a majority of Americans until 1950, amid post-World War II economic expansion and the baby boom.

Among the struggling rural areas are vast stretches of West Virginia in Appalachia. Several of the state's counties over the past decade have lost large chunks of their population following the collapse of logging and coal-mining industries during the 1960s.

In Moundsville, Frees describes his town, which sits in the northern panhandle along the edge of Pennsylvania near Pittsburgh, as appealing in some regards because of its low cost of living and friendly atmosphere in which "people talk to each other." But opportunities are few for the area's young adults other than perhaps the $7 or $8-an-hour jobs at the nearby Wal-Mart store.

"The young kids today are fleeing the area," Frees said. "They get the education and then they leave because there's nothing here for them."

Other rural U.S. counties suffering big declines include Issaquena, Jefferson and Sharkey in Mississippi; Sheridan and Towner in North Dakota; Kiowa in Kansas; Cimarron in Oklahoma; Tensas Parish in Louisiana; Monroe in Arkansas and Cottle, King and Culberson in Texas. All had percentage losses of 20 percent or more over the past decade.

The numbers are based partly on an analysis by the Population Reference Bureau. The data were supplemented with calculations by Robert Lang, a sociology professor at the University of Nevada-Las Vegas, and William H. Frey, a demographer at the Brookings Institution. "Rural" is generally defined as nonmetropolitan areas with fewer than 50,000 people.

While rural America shrinks, larger U.S. metro areas have enjoyed double-digit percentage gains in population over the past several decades. Since 2000, metros grew overall by 11 percent with the biggest gains in suburbs or small- or medium-sized cities. In fact, of the 10 fastest-growing places, all were small cities incorporated into the suburbs of expanding metro areas, mostly in California, Arizona and Texas.

In all, the share of Americans living in suburbs has climbed to an all-time high of 51 percent. Despite sharp declines in big cities in the Northeast and Midwest since 2000 due to the recession, U.S. cities increased their share by 3 percentage points to 33 percent.

"These new patterns suggest that there will be a blurring of boundaries as regions expand well beyond official government-defined definitions," Frey said. "People like to `have it all' — affordable housing in a smaller-town setting but in close proximity to jobs and big-city amenities such as specialized shopping, cultural events and major sports and entertainment venues."

"Many moderate-sized metro areas can fulfill all of these needs," he said.

The Census Bureau will soon begin to define new "combined statistical areas" — often referred to by demographers as megapolitan areas or megalopolises — based on growth and overlapping commuter traffic. Some analysts point to a merger of areas between Austin and San Antonio, between Tampa and Orlando and possibly between Phoenix and Tucson, with the Washington-Baltimore region extending southward to Richmond, Va.

These new megalopolises could help spur corporate and government investment in major cities and the growing small towns in between.

"There's such a large share of population that is now in reach of a substantial metropolitan center due to transit systems and highways, that the traditional notion of small-town America is changing," said Lang, who has done extensive research on U.S. megapolitan and regional growth.

"Fewer and fewer people live in the deeply rural places, and for most people in smaller towns, a big regional hospital or a Wal-Mart or strip mall is not too far away," he said.

He and other demographers believe that rural areas will remain viable, although many will be swallowed up by booming metropolitan areas and linked into sprawling megalopolises. Far-flung rural counties boasting vacation and outdoor recreation also will continue as popular destination points for young couples, retirees and empty nesters.

Lang said he hoped the growing convergence of major metro areas — and smaller towns in between — will promote better regional planning and cooperation rather than leading to individual cities acting as rivals for new investment. He said such collaboration might mean development of more roads or regional high-speed rail, or new approaches to water and energy conservation in the Mountain West.

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Associated Press writer John Raby in Moundsville, W.Va., contributed to this report.