But for all its drawbacks, Mai says she couldn’t be happier. She walks to work at a local publisher, eats out five times a week, and thinks of Boston Common as an ideal front yard.

“It hasn’t crossed my mind to ever want to leave the city,” said the 25-year-old Mai, who shares the 450-square-foot apartment with her boyfriend. “I’ve never thought of our place as too small. I really don’t need a big kitchen or a garden.”

Mai is among the thousands of young professionals whose devotion to urban living is causing Boston to grow at its fastest rate in decades. The influx has spawned a sweeping transformation of the city, with new residences and office buildings filling the skyline and reinventing commercial districts that once felt hopelessly time-worn.

Almost everywhere you look, it seems, is a new building site: A dozen towers are rising in the downtown area, and city-wide some 5,300 homes are currently under development. Boylston Street near Fenway Park is humming with construction during the day and crowds of diners at night. Downtown Crossing has lured fine restaurants and hundreds of luxury residences. And even once rough-hewn neighborhoods such as South Boston are increasingly drawing gourmet food stores, hip bars, and tony apartments.

The population surge has thoroughly reversed the suburban migration that began in the 1950s, when Boston peaked at about 800,000 people. Head counts in the South End and downtown have jumped by 20 percent since 2000.

In just one year alone — 2010 — Boston’s population grew by 7,500 people, and is now above 625,000, its highest level since the 1970s, according to city data.

Though largely driven by the generation between 20 and 34, the city is also swelling as empty-nesters trade suburban homes for urban pied-a-terres, and more young families opt for Boston over the traditional move to the suburbs in search of better schools. Regardless of background and interests, these people are drawn by the convenience and energy of busy urban neighborhoods.

“I like the feeling of being surrounded by people,” said Ece Gulsen, 27, who grew up on Turkey’s Mediterranean coast and now lives in the Charlestown Navy Yard to be near the water.

Companies are also moving into Boston to attract talented workers, developers are responding with even more housing, and restaurateurs, sensing a growing appetite for inventive food and entertainment, are opening eateries in places that defy conventional wisdom.

During the last two decades, Brad Fredericks, proprietor of Fajitas & ’Ritas, has watched the changes wash through the city’s downtown, where he recently opened a new eatery, the Back Deck grill on West Street. First, he said, Suffolk University and Emerson College expanded. Then the Boston Opera House was renovated, and the Paramount and Modern theatres reopened. Businesses then formed an association to help improve the area, and hundreds of apartments and condominiums are now under construction.

“I’m more optimistic than I’ve ever been,” he said. “There have always been a lot of activity generators down here. But the crowds have become more consistent. You see more people strolling around.”

The Boston real estate market is one of the strongest in the country, according to the Urban Land Institute, in part because of its strong housing market and the medical and technology companies who want to be near its population of highly educated 20- to 34-year-olds.

Maureen McAvey, a ULI fellow who specializes in retail development, said young professionals have particular preferences for housing, shopping, and travel that dictate how a city grows. For one, they are more willing to live in small spaces. They don’t feel the need to own a car, and make more frequent shopping trips.

“From a consumer standpoint, we’ve seen a large increase in people buying food on a two- to three-day basis,” McAvey said. “This generation wants the access and convenience that the city provides. They are much less interested in having a big lawn.”

But it is not just young workers queuing up for the city. Dick Reynolds, 67, relocated with his wife to a two-bedroom condominium in the South End when their kids moved out of their old home in Needham.