New York City officials agreed on Tuesday to a plan that would eliminate the use of solitary confinement for all inmates 21 and younger, a move that would place the long-troubled Rikers Island complex at the forefront of national jail reform efforts.

The policy change was a stark turnaround by the administration of Mayor Bill de Blasio, which recently eliminated the use of solitary confinement for 16- and 17-year-olds but, backed by the powerful correction officers union, had resisted curtailing the practice more broadly.

Even the most innovative jails in the country punish disruptive inmates over age 18 with solitary confinement, said Christine Herrman, director of the Segregation Reduction Project at the Vera Institute of Justice. "I've never heard of anything like that happening anywhere else," she said, referring to the New York City plan. "It would definitely be an innovation."

The Correction Department has faced repeated criticism over the past year after revelations of horrific brutality and neglect of inmates at Rikers, the country's second-largest jail system. Preet Bharara, the United States attorney for the Southern District of New York, is suing the city over the treatment of adolescent inmates at the jail complex.

Jail reform advocates who have criticized the Correction Department for years praised the initiative. The New York Civil Liberties Union said the change would "make Rikers a leader in solitary confinement reform."

"With these reforms, New York City has taken an important stand for basic human rights and reaffirmed its commitment to the safety of prisoners, prison staff and our communities," said Donna Lieberman, the organization's director. "An institution as profoundly broken as Rikers Island will require wholesale reform to transform into a humane environment that emphasizes treatment and rehabilitation over punishment and isolation, and these rules are a major step forward."

Norman Seabrook, president of the 9,000-member correction officers' union, said the plan, which appeared to take him by surprise, would endanger correction officers, leading to more inmate attacks. He vowed to sue the board for every guard assaulted.

"I'm deeply, deeply bothered by a decision that you would make, jeopardizing the safety and security of inmates and officers," he told the board on Tuesday.

Union officials said they felt excluded from the discussions about solitary confinement.

"It's a blindside," said Sidney Schwartzbaum, who leads the union for assistant deputy wardens.

"Eighteen- to 21-year-olds are a very violent group," said Mr. Schwartzbaum, who added that he had not met with Mr. Ponte since mid-December. "If we can't secure them, violence is going to go on unabated. What do we do with a guy who slashes someone's throat?"

After years of wrangling between inmate advocates and city officials over solitary confinement, the agreement on Tuesday appeared to have been worked out in a private meeting between Mr. Ponte and Ms. Hamill. The two met on Monday morning at the behest of Mr. Ponte and spent 45 minutes developing the details of the initiative.

Along with the changes to solitary confinement, the new initiative allows the department to open a new 250-bed housing unit for the most violent inmates, known as enhanced supervision housing.

The unit will hold inmates with a history of violence, including those affiliated with gangs and who have assaulted officers or other inmates. They will be locked in their cells for 17 hours a day, rather than the standard 10 hours.

Effective immediately, the new rules will reduce the maximum amount of time inmates age 18 and older can be sentenced to solitary confinement to 30 days, from 90. The department also will eliminate so-called owed time. In the past, inmates who left Rikers before completing their stint in solitary confinement returned there if they went back to the jail.

"For years, New York City has locked people up without the provision of adequate programs and treatment to change their thinking and their behavior," Mr. de Blasio said in a statement. "We are pursuing evidence-based practices that will lead to a safer and more humane system."

Speaking to reporters after the meeting, Mr. Ponte said he was confident he would be able to get the necessary resources from the city to put the plan in place next year. Ultimately, he said, getting young inmates out of solitary confinement will drive down violence.

"The idea we lock people up for any length of time and don't provide them with programs or treatments" does not lead to good outcomes, he said. "It seems to defy logic."