Mandatory sentences of life in prison without parole for juvenile offenders violate the US constitution, the Supreme Court has ruled.
The court on Monday threw out the life sentences of two men convicted as boys of murders in Arkansas and Alabama.
In both cases, state law mandated judges to impose life without parole.
In a 5-4 decision, the court ruled mandatory life sentences for juvenile convicts violated the constitution's bar on "cruel and unusual punishment".
"By making youth (and all that accompanies it) irrelevant to imposition of that harshest prison sentence, such a scheme poses too great a risk of disproportionate punishment," Justice Elena Kagan wrote for the court .
Justice Kagan was joined in the majority by Justices Ruth Bader Ginsburg, Stephen Breyer and Sonia Sotomayor, who are considered the court's liberal wing, along with Justice Anthony Kennedy, who is typically seen as a swing vote.
In 1999, Kuntrell Jackson, then 14, and other boys were trying to rob a video store in the state of Arkansas when another boy shot and killed a clerk.
'Not our decision'
Jackson was tried as an adult and convicted of capital murder. Under Arkansas law, he received a mandatory sentence of life imprisonment without parole.
Evan Miller was 14 in 2003 when he and another boy in Alabama beat a man with a baseball bat and set his trailer on fire, killing him. He was convicted of murder in the course of arson and received a mandatory sentence of life in prison without parole.
The court on Monday declined to go a step further and categorically bar all sentences of life without parole for juveniles.
In a dissent, Chief Justice John Roberts wrote that mandatory life sentences for juveniles are so common across the US that the sentence cannot reasonably be considered "unusual" and thus unconstitutional.
"Our society has moved toward requiring that the murderer, his age notwithstanding, be imprisoned for the remainder of his life," he wrote .
"Members of this Court may disagree with that choice. Perhaps science and policy suggest society should show greater mercy to young killers, giving them a greater chance to reform themselves at the risk that they will kill again. But that is not our decision to make."
Justice Roberts was joined by Justices Antonin Scalia, Clarence Thomas and Samuel Alito, who form the court's conservative wing.