CHARLESTON, S.C. –- Growing up in this starchy historic city in the 1990s, Jessica Duggan remembers field trips with her mother to the historic Battery neighborhood, watching tourists "doing the horse thing and the market thing." She dreamed of staying here as an adult, but had to admit: Her hometown was hopelessly uncool.

Fast-forward more than a decade and you'd hardly recognize the place. A booming tech start-up economy and a thriving arts and restaurant scene have helped this old Civil War tourist magnet do something that places across the USA have been trying to do for decades: attract young, college-educated workers and keep them there as they start families. The mild weather and easy access to nearly 200 miles of beaches don't exactly hurt.

"I always knew I wanted to end up here," says Duggan, 23. "It becoming cooler is a plus."

Charleston now teems with college-educated young people, 20- and 30-somethings who have come for the jobs and stayed for the lifestyle. New bars and restaurants seem to open weekly. Average commute times hover around 10 minutes. At the gas station on the way home, you can fill your growler with craft-brewed beer.

This is a new kind of city, born of deep demographic shifts and the power of technology. Where traditional college towns have long attracted young people who get an education and then leave, another kind of town is emerging: the post-college town.

Charleston is one of the smaller cities in this emerging brand of urban center. It joins larger ones such as Seattle, San Francisco, Boston, Minneapolis and Washington, D.C. They're 20-something magnets that don't just survive but thrive. Among those with the highest ratio, several are old-fashioned cities such as Charleston, and Alexandria, Va., places designed before automobiles arrived. Several of the most popular cities have become an important part of New Urbanism, which models development around mixed-use development and pedestrian-friendly spaces.

"These places seem to be built for people, not for automobiles," says University of Nevada-Las Vegas demographer Robert Lang. "And the 20-somethings love the people, not the automobile."

Using recent U.S. Census data, USA TODAY has identified 289 cities that have more 20-somethings than teens –- in the case of Charleston and about a dozen other cities, it's 2-to-1 or higher.

The higher the ratio, the stronger the local pull for young adults. That's key, because city residents who are ages 10 to 19 mostly grew up there. But those who are 20 to 29 are much more likely to have moved to a city to attend college, follow a boyfriend or girlfriend, get married or relocate for a job. A high ratio is also an indicator that many young people simply never left.

For Duggan, it's the latter. She attended the College of Charleston and graduated in 2011. At 23, she's already married –- she and her husband have an English bulldog named Winston, and she's editorial director at BiblioLabs, a small tech start-up that designs easy-to-navigate e-book lending websites for public libraries. The company, founded eight years ago, employs 30 people, many of them software engineers. The median age hovers in the mid-20s, and several of her colleagues also say they're here to put down roots.

WHERE THERE ARE JOBS ...

Alex Summer, a software developer from Newberry, S.C., bought a three-bedroom house in suburban Mount Pleasant in 2009, and jokes that he's "settling down" at age 27.

"You've got the beach; you've got the history; you've got the tech opportunities," he says. He's also engaged, so he now hangs out at "the more chill spots" downtown.

Eric Bowman, founder of local software start-up Sparc, says that among his 140 developers, the median age is about 28. And most of them know more than he ever did at that age about software. "These 25-, 27-, 28-year-olds are just blowing me away." In the past six months nine employees have left to start their own companies in town. "Every one of our developers can get a job in five days," he says, "so you have to treat your team members fairly."

Start-ups haven't been the only ones snapping up educated workers here. Boeing is expanding rapidly. The aerospace giant now assembles 787 Dreamliners at a rate of three a month at a massive facility adjacent to the Charleston airport, and over the next three years it plans to hire about 600 more information technology employees, bringing its total number of workers in the region to about 8,000.

City planners in Alexandria, Va., which ranks third in USA TODAY's post-college town ranking, have pushed to integrate commercial development, land use and transit to create a city that allows residents to "live, work and play in the same space," City Manager Rashad Young says.

When Young thinks of the young professionals in Alexandria, he says he imagines they're thinking, "I want activity, energy, vitality –- I want to be able to get places quickly. I want there to be a concentration of activity that I don't necessarily have to drive to get to. I want easy access to places and things."

The demographic change in Charleston has opened up previously sketchy neighborhoods for development, a change that for many locals has been nothing short of breathtaking.

"The whole face of this city has changed in a year-and-a-half," says David Crowley, a co-owner of The Alley, a popular sports bar that boasts eight lanes of bowling. It's located in a rapidly changing industrial area about a mile north of downtown, and Crowley says the enterprise would have been unimaginable until recently.

When he attended classes at the College of Charleston a decade ago, "You didn't go north of Calhoun (Street). It wasn't safe." Now he gets so much business most nights he's got to lease 85 parking spaces from _The (Charleston) Post and Courier _across the street.

Crowley and his partners built the venue from scratch inside an old liquor-distribution warehouse, and on a recent weekday evening a family with young children rented bowling shoes while a young couple, both of them 31, held a wedding rehearsal dinner in an upstairs event space. In between, an early-evening crowd drank beer, watched live golf on big-screen TVs and played retro coin-operated video games. "They're in Charleston to live in Charleston," says Crowley.

WORK, OR KITE-SURF?

Two miles north on Meeting Street, workers are putting the finishing touches on a 13,000-square-foot renovation of the long-vacant 1926 Standard Oil regional headquarters building that soon will be home to about a dozen small creative businesses.

"This is truly what I consider the last frontier of the Charleston peninsula," says Lindsay Nevin, a young developer who is working with the city to build a "creative corridor" on Meeting Street, an industrial thoroughfare once dominated by car dealerships. It's already dotted with small artists' storefronts and independent restaurants. A small-batch distillery recently opened down the street, and Nevin, who bought the abandoned office building last June, has signed three-year leases with, among others, a glass sculptor, a potter, a photographer, a dressmaker, an art magazine and two interior design firms.

Lisa Maki, a co-founder of the tech start-up PokitDok –- it helps consumers find low-cost health care providers –- says 15 of the company's 23 employees are based in Charleston. All 15 are software engineers. She calls them "our secret weapon" and says she can hire good engineers here for about half the cost of comparable workers in Silicon Valley. Maki says office space here is "probably a quarter of the price" of comparable space in Silicon Valley.

Many employees want to raise their children in a less high-stress environment, and they love the ability to live close to work, she says. "If the wind comes up, our kite-surfers head to the beach and then get online later. That's just what they do," Maki says.

Much of the region's development owes its success to both public and private investment and upgrades in infrastructure. Government incentives helped to attract and keep companies such as Sparc. Projects such as the 2005 Cooper River Bridge, which connects downtown Charleston and Mount Pleasant, have helped outlying areas thrive. The bridge replaced a pair of notoriously narrow crossings.

The area it helped open up now is home to some of the area's major employers, including health-tech giant Benefitfocus, which opened in 2000 in a shuttered Walmart. When CEO Shawn Jenkins and a partner started the company, he says, "People were telling me that you could never build a tech company here." They're now in the midst of an huge expansion at their headquarters east of downtown, which will add 1,200 more employees by 2015.

The company, which went public last September, develops software that helps employees manage workplace health and life insurance benefits. It has an estimated 20 million users.

"I was always a believer that this town was going to be awesome," Jenkins says. He jokes that his recruiters often wait till it's snowing elsewhere in the USA to try tempting prospective workers to visit.

Once they're at work, his "associates," as he calls his employees, enjoy coffee from free Starbucks dispensers and Coke machines rigged to dispense drinks for 25 cents. One floor of their campus has been remodeled as a "social work space" that resembles an open-floor loft or a high-end hotel lobby. Hoping to inspire his Web developers to design beautiful stuff, Jenkins has peppered the offices with handsome objects, including a small collection of Fender Stratocaster guitars and, in the middle of one workspace, a gleaming red Ducati 1199 Panigale motorcycle.

Speaking of design, Charleston may be "the single most important city of inspiration to the New Urbanists" who have pushed to redesign cities around more densely populated, pedestrian-friendly living spaces, says Lang, the demographer.

He's seen it before, in places such as Alexandria, Va., and Savannah, Ga., both of which rank high on USA TODAY'S post-college list. "They're young and they live in old cities," says Lang. "Some of America's oldest cities have the youngest population."