Nationwide, the age-65-and-older population increased by 15 percent between 2000 and 2010, according to a recent Brookings Institution analysis of 2010 Census Bureau data. But many major U.S. cities are aging much faster than that. Some 31 metro areas, primarily in the South and West, have seen their senior-citizen populations increase by more than 25 percent over the past decade.

The proportion of those age 65 and older in Raleigh-Cary, N.C., has increased by 60 percent since 2000, the largest uptick of any metro area in the United States. And three Texas cities, Austin, Houston, and Dallas, are among the places with the fastest-growing senior citizen populations. Many cities out West are also home to rapidly growing retiree populations, including Las Vegas, Nev., Boise, Idaho, Provo, Utah, and Colorado Springs, Colo. Only five metro areas registered declines in their numbers of senior citizens: Scranton, Pittsburgh, New Orleans, Buffalo, and Youngstown.

Often, people are choosing to spend their retirement years in places where they spent the final years of their career.

"Most of the country is increasing its over-age-45 population simply because people are aging in place," says William Frey, a Brookings Institution senior fellow and author of the report. While some people move to designated retirement communities once they leave their jobs, most people don't. "There is some movement among the retired population, but it's not that huge and people who move typically move locally," says Frey. "Florida is the outlier. In the rest of these places the migration is occurring among people under age 45."

The suburbs of the nation's largest metro areas are aging faster than the cities. Baby boomers and seniors are now more likely to live in the suburbs than young families, and the child populations in many suburbs have decreased since 2000. Some 40 percent of the suburban population is age 45 and older, up from 34 percent in 2000. "The baby boomers were the first suburban generation. They were born in the suburbs and came back after college and raised families there," says Frey. "Now the suburbs are aging with them."

As the earliest baby boomers begin to turn 65 this year, many cities will soon be coping with even bigger retiree populations. The number of people age 55 to 64 has increased by 50 percent over the past decade, reflecting the ascension of the early baby boomers into this age range. Since most people retire near where they spent their career, the cities with large numbers of older workers are likely to see an uptick in their retiree populations in the near future.

The number of pre-seniors between ages 55 and 64 has increased by 97 percent in Raleigh-Cary, N.C., and it has more than doubled in the Austin-Round Rock, Texas, area since 2000. "The fact that Texas is better off economically than almost every other urbanized part of the country is part of the reason that many people are coming here," according to Ryan Robinson, Austin's city demographer. "Austin has been extremely attractive for families with children who stay here, and that's why we're starting to see the slightly older folks."

Many areas with the fastest-growing populations of older workers in their late 50s and early 60s are college towns, including Colorado Springs, Colo., Provo, Utah, Madison, Wisc., and Albuquerque, N.M. "These are places that attracted people there during their younger years," says Frey. "Aging in place is the primary growth engine of the senior population in most parts of the country."