Washington, D.C. ? The Supreme Court took two cracks at one of the law’s thorniest questions Monday: When can you lock up a prisoner and throw away the key?

Not when it’s a teenager who hasn’t killed anyone, the justices said. But when it’s a “sexually dangerous” inmate, maybe so, even if he has completed his federal prison sentence.

By a 5-4 vote, the court said young people serving life prison terms must have “a meaningful opportunity to obtain release” if they haven’t killed their victims. The majority opinion by Justice Anthony Kennedy extended the “children are different” rationale that drove his decision five years ago that outlawed the death penalty for killers under 18.

The court ruled in the case of Terrance Graham, who was implicated in armed robberies when he was 16 and 17. Graham, now 23, is in prison in Florida, which holds 60 percent of juvenile defendants who are locked up for life for crimes other than homicide.