Massachusetts cannot hold juvenile offenders for three years past their 18th birthday because the law is vague on how a youth's potential danger to the public is decided, the state's highest court ruled Tuesday.

Most juvenile offenders are released when they turn 18, but the extended commitment law allowed the Department of Youth Services to retain custody up to age 21 if a youth posed physical danger.

In unanimously striking down the law, the Supreme Judicial Court said it was unconstitutional because the law "contains no indication of the nature and degree of dangerousness that would justify continued commitment."

The court noted that the Legislature in 1990 had removed language that the danger a youth posed be due to a mental condition.

"The court said we don't lock people up based on an allegation of dangerousness when we don't know what that means," said attorney Barbara Kaban, deputy director of the Children's Law Center of Massachusetts.

She represented the three youths who challenged the law. Kaban said the three had been charged with crimes that included larceny and assault and battery with a dangerous weapon.

All were turned over to the department's custody when they were 16. Each was initially placed on probation, then sent back to a juvenile detention facility after violating the terms of probation at least once. The department filed applications for extended commitment orders for all three shortly before they turned 18.

Jennifer Kritz, a spokeswoman for the Department of Youth Services, said the department is reviewing the court's ruling and expects to be ordered to release about 12 youths being held under the law.