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.