“Our neighborhood is one of the last neighborhoods left in New York where you have these big old Beaux-Arts buildings, built for wealthy families,” Ms. Falcon said, referring to the stretch of Harlem from 145th to 155th Streets near the Hudson River. She said groups of adults, each contributing, pay rents that families cannot or choose not to pay.

Image Wil Fenn, 29, pays $850 a month for a room in a four-bedroom apartment. Credit... Jennifer Altman for The New York Times

New York City has long been a magnet for the young, well educated and ambitious. According to a report published by the Census Bureau in 2003, nearly 132,500 young, single, college-educated people poured into the New York metropolitan area between 1995 and 2000, more than into any other metropolitan area in the United States.

“Sometimes we underestimate how important that is in generating the city’s creativity,” said Frank Braconi, chief economist for the city comptroller’s office. “To the degree that housing costs become a barrier to that group, it can in the long run sap us of that creative potential that we would otherwise have.”

Brad Lander, director of the Pratt Center for Community Development, a nonprofit group, said young professionals get less attention than other financially struggling groups because they are more mobile and have options. Though they, too, are wrestling with the city’s shortage of lower-cost housing, they are seen as harbingers of gentrification.

Mr. Lander said a well-known strategy among landlords of buildings with rents regulated by the city is to seek out tenants who they imagine will not stay long, because they can often increase the rent when a tenant leaves. “Students as well as professionals,” he said. “Plenty of landlords find this group an attractive set of folks to rent to, believing they’ll be out in a couple of years.”

Marieke Bianchi, 23, a junior account executive at a public relations firm in the Flatiron district, moved to New York from St. Louis last year after graduating from college. She started out on a friend’s couch, then sublet for six weeks in Hell’s Kitchen, where she had to move a giant exercise bike to get into bed.