Deprived of their parents' care by death, prison, drug addiction or other causes, they go it alone, even though that often means finding their own food and shelter.
Many are fearful of the remedies governments typically provide, having heard horror stories from their peers about foster care or group homes.
"It points out a gaping hole in what is available and the drawbacks of the child welfare system," said Nan P. Roman, president of the National Alliance to End Homelessness. "They are no one's responsibility."
The answers for homeless adults, Roman said, are more straightforward. They can get housing subsidies. But teens require not just a safe place to live but guidance from responsible adults and other services that will help them build a stable life as well.
"They get caught in an adult system that doesn't work for them and a juvenile system that doesn't work for them," Roman said.
Identifying them is a vexing problem. Locally, the city and county departments of social services are charged with protection, but the children often won't come forward, and schools, police and social agencies don't actively look for them.
"If a friend or relative does not bring the situation to our attention, they will go undetected by our agency," said Samuel Chambers Jr., director of the Baltimore Department of Social Services.
Among its 88,000-student enrollment last year, the Baltimore school system reported 1,339 as homeless at some point, the majority of whom were living in shelters. Principals reported last year that they suspected another 950 children were likely homeless but could not verify that. Once identified, they were eligible for services such as transportation, school uniforms and tutoring. Younger ones were invited to a summer camp, and teens were able to take part in a work-study program set up by the school system.
Only 189 of the schoolchildren identified as homeless were high schoolers, said Louise Fink, who heads the city schools' program for homeless children. She said that officials believe hundreds more homeless high school students are uncounted, because they are ashamed or afraid to admit they are on their own.
"I feel very strongly that those high schoolers should be the first focus of our attention," she said.
They have fewer options, she said, because most shelters won't take them. And if you are a student who doesn't finish high school, she said, "you may doom yourself to poverty."
The one shelter for such teens is operated by Fellowship of Lights, a nonprofit.
The problem of teen homelessness is gaining attention, but few solutions have emerged. "We haven't figured out how to try to tackle that," said Greg D. Shupe, director of the office of transitional services in the state Department of Human Resources.
In Baltimore during the past year, several nonprofits have discussed ways to assist homeless and runaway teenagers, perhaps by finding a site for a drop-in center. There, they could launder their clothes, take a shower or spend a night.
The social services department, which is a part of DHR, has begun to consider a similar idea.
But no one has figured out how to negotiate the practical, legal and ethical dilemmas in such a plan: How do you entice homeless teens who fear being forced to accept more help than they want? And if you do identify them as homeless, how could you let them return to the streets?
Furthermore, many teens regard the help that Social Services provides as nightmarish. They have heard stories of residents in group homes being beaten and robbed. Foster care strikes them as no better, a forced separation from neighbors, friends and familiar schools.
One way to reduce the number of teens living on their own, Chambers said, would be to increase the number of foster homes in the neighborhoods where the teenagers live.
No comprehensive national study has quantified how many teenagers live on their own without stable housing, homeless advocates say.
For two years, federal law has required school systems to provide services to homeless children and to report the number of children in shelters or who are "doubled-up," a term that describes those moving from house to house, with or without a parent, or who could be living in a car or a vacant house.
In Maryland last year, school systems reported about 6,500 children who were homeless or doubled-up. In Virginia, about 7,000 children were identified, said Patricia Popp, president of the National Association for the Education of Homeless Youth. The figures across the nation, she said, are really estimates as schools systems get used to compiling them.
In cities, the numbers range from about 10,000 students in Philadelphia with an enrollment of 250,000 to 1,325 in the Cleveland public schools, which enroll 65,000 children.
"Kids are amazingly resilient, and they will find a way," said Ross Pologe, executive director of Fellowship of Lights, the nonprofit that runs the only teen shelter in Baltimore. But "it is hardly a smooth path, and the downside risks can have frighteningly tragic results."