QUINCY -- Two months before he turned 19, Tom Henry got an ultimatum: He needed a job, a driver's license, and $2,000 toward an apartment by his birthday.

It wasn't the gentle nudging of a resolute parent; it was the harsh reality of ''aging out" of state custody. After growing up in state programs for children with behavioral problems, Henry was expected to slip into adulthood smoothly -- a transition that's often rough even for teenagers with supportive families and college educations.

Henry failed to get a steady job or apartment and ended up ''couch-surfing" at friends' homes until he landed at Father Bill's Place, an adult homeless shelter in Quincy. He's now 24 and in his second stay at Father Bill's.

All of which makes Tom Henry the symbol of a problem that is attracting new attention, nationally and locally.

''This phenomenon of [homeless] young adults -- it's not only young people out of foster care, young people aging out of the Department of Youth Services. There's a lot of runaways and throwaways who never made it. . . . It's scandalous and it's disgraceful and it's horrific -- and it's real," said Philip Mangano, the Bush administration's point person on homelessness and former head of the Massachusetts Housing and Shelter Alliance. ''Across the country, I hear more and more people talking about" it.

In Massachusetts, the most recent statistics show that the number of young adults -- defined as those aged 18 through 24 -- who were new to homeless shelters tripled between 1997 and 2000, and the upward trend continues, according to the Massachusetts Housing and Shelter Alliance. Young adults made up almost a fifth of the state's new homeless population in shelters in 2000, according to a study by the agency.

Locally, the number of homeless young adults served by shelters in Quincy and Brockton grew by 28 percent over the last year.

At Father Bill's Place in Quincy, 205 young adults were served during the fiscal year 2005, compared to 154 the previous year, and up from about two dozen a decade ago. Brockton officials saw 58 young adults in the last fiscal year, compared with 51 a year earlier.

''Some . . . [are] graduating out of Department of Youth Services programs; some people have not completed their schooling and are struggling in the job market," said Dennis Carman, executive director of the Mainspring Coalition for the Homeless in Brockton.

''The sad part is, it's a young age, an impressionable age."

Homeless young adults make up their own category, separate from the broader ranks of homeless families with young children, and the increase in their number has been blamed on a variety of factors, including the breakdown of traditional family structures, a lack of funding for substance abuse programs and inadequate support institutions, and the high cost of housing.

As cities such as Quincy launch much-trumpeted 10-year plans to end chronic homelessness by providing more and better housing, the rise of youth homelessness is seen by some as signaling a new generation of people living on the streets.

''This could be our future chronic [homeless] group. That's what I get concerned about," said John Yazwinski, executive director of Father Bill's Place. ''If we put so much attention on the chronic, [what will happen to] these new populations?"

It is a fluid population, and often hard to track.

In discussion groups, homeless young adults often talk about making plans as matter-of-factly as other kids their age -- only the content is different. ''They'd think: 'I'll go into the emergency shelters; stay outside during the summer; I'll head south for the winter; come back and commit some petty crimes and go into jail' -- just as matter-of-fact" as any other young adult mapping the future, Mangano said.

On a recent night, a handful of young adults wearing sweatshirts, baseball caps, and wary, drained looks milled around the shelter.

In interviews, six men who ranged in age from 19 to 24 told overlapping stories. Some had been jolted out of the constant supervision of state custody into the wider world with no safety nets or preparation; others had been tossed out of, or run away from, broken families; some had problems with drugs; others had problems finding work; some suffered from mental illness.

Few had told their friends where they were living.

As they talked, the men swapped tips about job training programs, inquired about old acquaintances at halfway houses, and commiserated about the fact that few could ever imagine going back home to their parents.

When asked to ponder their futures, none saw themselves back at the shelter -- and yet Tom Henry and John Frost, 24, were both on their second stint at Father Bill's, having spent time there years before.

They praised Father Bill's as a great place to live when things went wrong: ''Let's put it this way -- it's like the Ritz, only it's better," Frost said.

Such praise is not what shelter workers want to hear.

''The last thing we want is them to get comfortable here," said Father Bill's shelter manager, Jill St. Martin. But she described a lack of other options. ''This population is very unique: There aren't enough agencies and service options. Sometimes we are referring them to programs that work with adults, but they are unique adults."

Henry described spending four months applying for 200 different jobs, without success. John Coveney, 19, of Scituate was about to spend his first night in the shelter but said he wasn't nervous, having been constantly shuttled among different treatment centers. Steve Sproul, 19, hunched low in his chair with a hat pulled down over his eyes, said that his life over the past four years was a blur. ''They're kind of in a blind spot of policy," Mangano said. ''Too old for the youth system, but definitely too young for an adult system."

To combat the problem, the Massachusetts Housing and Shelter Alliance plans to advocate for money in the state budget to help move young adults rapidly from shelters into housing. Projects such as Bridge Over Troubled Waters try to cater specifically to the needs of the younger crowd. Several of the men interviewed expressed an interest in federal programs, such as Job Corps, which train people for careers.

Advocates say that part of the problem is within the shelter itself, where neither the housing nor the supportive services are strong enough.

''They're at that age of in-between," Yazwinski said. ''More and more kids are not having direction and struggling without education and trying to deal with mental health issues . . . and they need that opportunity to be kids."

Carolyn Y. Johnson can be reached at cjohnson@globe.com.

Out in the street

Young adults, ages 18-24, in Massachusetts homeless shelters for the first time.

1997 839
1998 1,278
1999 2,383
2000* 3,144

*Most recent figures available

Source: Massachusetts Shelter and Housing Alliance