Correction: An earlier version of this article misstated the proportion of people in Virginia and Maryland who are 45 or older. It is four in 10, not one in four. This version has been corrected.

The nation’s suburbs are home to a rapidly growing number of older people who are changing the image and priorities of a suburbia formed around the needs of young families with children, an analysis of census data shows.

Although the entire United States is graying, the 2010 Census showed how much faster the suburbs are growing older when compared with the cities. Thanks largely to the baby-boom generation, four in 10 suburban residents are 45 or older, up from 34 percent just a decade ago. Thirty-five percent of city residents are in that age group, an increase from 31 percent in the last census.

During the past decade, the ranks of people who are middle-aged and older grew 18 times as fast as the population younger than 45, according to Brookings Institution demographer William Frey, who analyzed the 2010 Census data on age for his report, “The Uneven Aging and ‘Younging’ of America.” For the first time, they represent a majority of the nation’s voting-age population.

The political ramifications could be huge as older voters compete for resources with younger generations.

“When people think of suburban voters, it’s going to be different than it was years ago,” Frey said. “They used to be people worried about schools and kids. Now they’re more concerned about their own well-being.”

The nation’s baby boomers — 76 million people born between 1946 and 1964 — were the first generation to grow up in suburbia, and the suburbs is where many chose to rear their own children. Now, as the oldest boomers turn 65, demographers and local planners predict that most of them will not move to retirement areas such as Florida and Arizona. They will stay put.

“If you ask younger boomers, who are 45-ish, a lot say they expect to move and retire elsewhere,” said John Kenney, chief of aging and disability services with the Montgomery County health department. “But as people get to 65 and 70, whether because of choice or default, they end up staying. We are planning on people being here.”

Local governments are starting to grapple with the implications.

As part of its 50 Plus Action Plan, Fairfax County has converted its pedestrian traffic signals to countdowns so people can gauge whether they have enough time to cross.

The county has held forums on kitchen and bath remodeling designs that make the areas accessible for wheelchair users. It collaborated with George Mason University in a course on coping during retirement. And a police unit has been formed to focus on financial fraud committed against the elderly.

About 1.5 million people, or 27 percent of the Washington area, are part of the baby-boom generation. The largest concentrations are in Fairfax, with about 310,000 boomers; Montgomery, with 275,000; and Prince George’s County, with 225,000.

“Clearly, the age wave is coming,” said Pat Herrity (R-Springfield), a county supervisor who heads the 50-plus committee.

In Montgomery, where 11 residents turn 65 every day, the county two years ago held a summit on the needs of its elderly residents .

Since then, it has developed a Web site listing all of the services available to seniors. It has helped several neighborhood groups establish “villages” in which younger residents volunteer to help drive their elderly neighbors to appointments. And to meet seniors’ growing need for income after the stock market faltered, it has staged job fairs.

“Retirement used to be the golden years,” said Kenney. “No more.”

According to the AARP, nine in 10 older Americans want to stay in their homes as they age, a figure the association predicts that the boomers will match. Not all communities are prepared.

“AARP research shows that most communities are behind in planning for their aging populations, but those that are adapting have come up with common-sense solutions to improve home design and make transportation easier,” said Nancy LeaMond, the AARP vice president, in a written statement.

Although Florida and Arizona remain retirement magnets, 17 of the 25 states with the highest concentrations of senior citizens are cold-weather states.

In the Washington region, the proportion of residents 45 and older rose almost five percentage points over the decade, from 32 to 37 percent. One in 10 residents is 65 or older.

In Maryland and Virginia, about four in 10 residents are 45 or older, up from about 34 percent in 2000. The District, in contrast, gained just a fraction of one percentage point of over-45 residents during the decade, going from barely 34 percent to less than   35 percent. It also was one of the few places in the country that saw a drop in the percentage of people who are 65 or older.

The census data mined by Frey for his report show how rapidly the nation is aging overall. The median age has increased in every state and almost every county, up from 32.6 in 1990 to 37.2 today.

Older Americans now represent 53 percent of voting-age adults.

“The political clout of older Americans will be even more magnified if the traditional higher turnout of this group continues, and as the competition for resources between the young and the old becomes more intense,” Frey writes.