Three years ago, the U.S. Supreme Court rightly struck down laws requiring minors who commit murder to be sentenced to life in prison without the possibility of parole. On Tuesday, the justices considered a different question: whether the decision should be made retroactive, which would require states to grant new sentencing hearings to prisoners who were younger than 18 when they committed their crimes. The case concerns Henry Montgomery, who was 17 when he shot and killed a Baton Rouge, La., police officer in 1963.

Montgomery vs. Louisiana involves the question of whether the Supreme Court's 2012 ruling was "substantive" (in which case it can be applied retroactively) or merely "procedural." It is complicated by an even more technical dispute over whether the Supreme Court even has jurisdiction over this state court issue. We hope the court focuses on the big picture: that its 2012 ruling is indeed a substantive change in the law that must be applied retroactively in all proceedings.

There is no doubt, as Justice Elena Kagan put it Tuesday, that the 2012 ruling "fits on the substantive side." Although the court in that decision stopped short of holding that juveniles could never be sentenced to life in prison without parole, it required judges and juries to consider "youth and attendant characteristics" as possible extenuating factors before imposing such a sentence. As a result, the court suggested, life sentences without the possibility of parole for juveniles in the future would be "uncommon."

That was almost as significant a change in legal doctrine as the court's 2005 decision holding that states couldn't execute murderers who were younger than 18 when they committed their crimes. Both rulings were based on the court's recognition that juveniles have "diminished culpability and greater prospects for reform" — an insight that reflects recent research on how adolescent brains function. It would be an injustice if prisoners such as Montgomery were kept behind bars simply because they committed their crimes before the court saw the light.

At Tuesday's argument, there were suggestions that the court might not use this case to decide whether its 2012 ruling is retroactive, deferring such a decision until a prisoner files suit under a federal habeas corpus statute rather than in state court. We hope the court doesn't dodge the question in that way, but sooner or later it will have to confront the question of retroactivity. When it does, the only just conclusion is that prisoners such as Henry Montgomery are entitled to a new day in court.

FROM AN EDITORIAL IN THE LOS ANGELES TIMES