The Court decided that judges could still sentence juveniles to life without parole in individual cases of murder. The ruling bars state and federal law from automatically imposing those sentences in what are known as "mandatory-minimums."
Twenty-six states and the federal government have made life in prison without parole mandatory for some types of murder and allowed it to be applied to 14-year-olds, court papers said. In addition, Louisiana has some mandatory life without parole sentences for 15-year-olds, and Texas has some for 17-year-olds.
The ruling continued its trend of holding that children cannot be automatically punished the same way as criminal adults without considering their age and other factors.
The 5-4 decision split along ideological lines: The court's four liberals and swing vote Justice Anthony Kennedy joined to order states and the federal government to allow judges and juries to consider a juvenile's age when they hand down sentences for some of the harshest crimes, instead of making life in prison without parole automatic.
Dissenting, the court's four conservatives said nothing in the Constitution forbids laws requiring mandatory life in prison without parole for juveniles. Chief Justice John Roberts was joined in the main dissent by Justices Antonin Scalia, Clarence Thomas and Samuel Alito. Alito also wrote separately and read his dissent aloud in the courtroom.
Alabama Attorney General Luther Strange expressed disappointment with the decision. "It is rare that a juvenile commits the worst kind of murder. But when this happens, the Alabama Legislature and most other American legislatures have determined that the appropriate sentence is life without parole. And they have made that sentence mandatory. Thus, it is hard to understand the Court's ruling that this sentencing procedure is `unusual' for the purposes of the Eighth Amendment," he said.
The court's ruling was based on the Constitution's Eighth Amendment prohibition against cruel and unusual punishment.
According to data provided to the court, roughly 2,500 people are behind bars for life with no chance of winning their freedom for murders they committed before their 18th birthday. More than 2,000 of them were there because the sentence was mandated by a legislature.
Advocates say 79 of them are in prison for crimes that took place when they were 14 or younger.
The cases were Miller v. Alabama, 10-9646 and Jackson v. Hobbs, 10-9647.
The Associated Press contributed to this report.