Residents used to pay well below market rates at Waterside Plaza, raising families and growing old in a middle-class Manhattan community of four residential towers on the East River. But rents soared and scores of tenants moved out after the development aged out of the state’s Mitchell-Lama program two decades ago. The surrounding Manhattan neighborhood changed, and today only about 28 percent of the apartments are still occupied by longtime residents.

Most of them are older than 70, retired and paying a burdensome percentage of their incomes in rent.

But they may soon get a respite.

Many of those residents would get a rent freeze, some even a rent reduction, as part of a deal struck this week between New York City and the property’s owner, Richard Ravitch, a real estate developer who also served as lieutenant governor and chairman of the Metropolitan Transportation Authority. The agreement — a rare happy outcome for renters in today’s real estate market — would preserve the affordability of 325 apartment units over the next 75 years and would effectively allow current residents to age in place in a section of Manhattan seeing increased development and an influx of young people.

“This is a really unique opportunity from a preservation perspective because it is an unregulated property in downtown Manhattan,” said Kim Darga, an associate commissioner of preservation at the city’s Department of Housing Preservation and Development.