When their daughter turned 18, Bill and Susan Louisignau gave her a charm bracelet and a pizza party.  Susan Louisignau looked on uneasily as her daughter passed the invisible line into legal adulthood. Along with the gifts, the girl had come into the rights accorded every adult in Massachusetts: to stop taking her psychiatric medication, to keep her treatment secret from her parents, and to quit treatment altogether.

Ten days later, their daughter, who has bipolar disorder, came home from school so manic that she threatened to burn herself alive in the family car. Over the next two years, she was hospitalized 11 times, at a cost to the state that her mother estimated at half a million dollars. It may not have been clear at the pizza party, but now Louisignau understands that the girl had moved, without support, into a territory beyond her control.

"When you turn 18, if you have this kind of diagnosis, you cannot be given carte blanche," said Louisignau, a real estate appraiser who lives in Greenfield. "I think that's where the breakdown comes in the law. You're all of a sudden telling a population that they're entitled to all the rights of adulthood."

In a mental health system with plenty of weak points, analysts are increasingly focusing on the period around a person's 18th birthday - a juncture where, they say, patients often take a disastrous turn toward homelessness, criminal behavior, and a revolving-door cycle of crisis, hospitalization, and release.  Beginning on their 18th birthdays, patients lose access to a broad array of state services, including mental-health case workers, day programs, and monitoring at schools or by probation officers. The changes come just as they are granted adult rights - including the right to decline medical treatment and to keep medical information from even their immediate family. Many do not qualify for adult services, and those who do often find them unappealing.

The disruptive changes that accompany an 18th birthday came up this year in two violent criminal cases. Jason Potter, 21, who allegedly stabbed his mother and stepfather to death in September, had decided when he turned 18 to stop taking his antipsychotic medication, his father said. And 20-year-old Jermaine Berry, a schizophrenic who allegedly shot two police officers Jan. 1, had been monitored by the Department of Youth Services until his 18th birthday.

For state services to terminate abruptly during the danger zone of late adolescence ends up incurring a great cost later on, as young people wind up in jails or hospitals, said a researcher at UMass Memorial Medical Center. And yet that's the way many programs are designed.

"If you could get them to age 25," most would have passed the danger zone, said Maryann Davis, who has become a national authority on late-adolescent youth. "It simply is an inopportune time to pull services."

The cutoff dates come one by one during high-risk years. At 17, a young person can be prosecuted as an adult. At 18, he or she gets the right to refuse treatment and DYS monitoring can end. At 19, juvenile services from the Department of Mental Health end. Gradually, data have been building to support what many social service professionals have believed for years - that young people who "age out" of juvenile mental health services often spin off on disastrous paths:

* One-third to one-half of adolescents receiving mental health services do not continue to do so as adults, according to Davis. This is largely because adult mental health services are geared toward severe mental illnesses like schizophrenia and bipolar disorder, which have traditionally been diagnosed after adolescence. Eligibility for children's services does not require such severe diagnoses, and many children with emotional disturbances are accepted.

* Children treated for mental illness are frequently arrested, prompting a "cost shift to corrections," said Davis, coauthor of "Transition to Adulthood: A Resource for Assisting Young People with Emotional or Behavioral Difficulties." The average age of service termination is 17, and for those who are arrested the peak time of arrest is between the ages of 18 and 20, according to a study by Davis of 131 juvenile DMH clients. Among that group, 64 percent were arrested and arraigned before their 25th birthday, more for nuisance crimes than violent crimes. Almost half were charged as adults.

* Many mentally ill young people end up homeless. Almost half of 323 homeless people ages 18 to 24 surveyed on one night this year had been treated in a psychiatric hospital, according to a census by the Massachusetts Housing and Shelter Alliance. One in 10 had spent time in a DMH facility, and 61 percent had taken psychiatric medication.

"It really does suggest that we need to be thinking differently" about the transition from child services to adult services, said Joan Mikula, assistant commissioner for child-adolescent services at the Department of Mental Health.

A year-and-a-half ago, at the pressing of consumer advocates, DMH helped fund a survey of young adults who had "aged out" of juvenile mental health services. In a report released this winter, the research firm Consumer Quality Initiatives found that many complained of a traumatic loss of personal support and no independent living skills training. Many young people find adult services unappealing, said Jonathan Delman, the firm's executive director.

"They see people who are stuck. They see the ravages of side effects of medication. They see age," said Delman, who suffers from bipolar disorder. "They're in a position where thay can think, `I can make it, maybe.' . . . You can't imagine what it's like to be in a day program or a clubhouse and being with people who you think you might become."

Delman's recommendations revolve around creating services that could be more appealing to young people, such as peer mentoring services, in which teenagers would be paired with older peers who had already been through the transition out of child services. Other programs in place, like Super Employable People in Quincy, coach young people on job interviews, socialization, cooking, and budgeting skills - training that "can be fun, and also maintain a sense of self-respect," said Katie Payson, the program's director.

What is not likely to change, said Mikula of DMH, are the state and federal laws that grant young people the right to refuse treatment and medication at the age of 18.

For some parents, those laws remain the central problem, giving their children veto rights just at the time in their lives when they most resist authority. If her daughter had been forced to move into a supervised group home, Louisiagnau said, the 11 emergency hospitalizations would not have taken place. "It doesn't make any difference right now how many programs we have," she said. "The law has got to change."

Even for parents who would not alter the laws, the transition can be jarring, even frightening. One night last fall, a case manager from the Department of Mental Health came to John and Carol Willett's home carrying a sheaf of release forms for their son, Michael, who had turned 18 a week before. Michael had been treated by a psychiatrist since he was 14, when his obsessive hand-washing became a problem, and was eventually diagnosed with schizoaffective disorder. As Michael's parents stood by, the case manager informed him that unless he signed the forms he no longer needed to inform them of any of his treatment decisions.

"I didn't know what to make of it all," said Michael Willett, who immediately signed the release forms because, he said, "I want my parents to be informed." But since then, it has gradually dawned on his parents how completely they could be shut out of their son's medical treatment.

"Right now, if he goes psychotic I can't do anything," said his mother. "That scares me silly."