Chief Justice John Roberts and Justices Antonin Scalia, Clarence Thomas and Samuel Alito dissented.
"Even a 17 ½ year-old who sets off a bomb in crowded mall or guns down a dozen students and teachers is a 'child' and must be given a chance to persuade a judge to permit his release into society," said Alito, who read his dissent aloud in the courtroom. "Nothing in the Constitution supports this arrogation of legislative authority."
In Utah, judges already have the option to sentence youth convicted of murder with a sentence other than life without parole, depending on the mitigating circumstances, said Christopher Ballard, assistant attorney general.
"I don't think Utah has ever had a circumstance where life without parole was mandatory," he said.
A Salt Lake Tribune analysis of prison records found there are only three men in Utah currently serving sentences of life without parole for crimes they committed as juveniles.
Ballard said the Supreme Court decision will only impact states where a sentence of life without parole was mandatory for youth, and those states will have to resentence all those offenders.
The decision came in the robbery and murder cases of Evan Miller and Kuntrell Jackson, who were 14 when they were convicted.
Jackson was sentenced to life in prison in Arkansas after the shooting death of a store clerk during an attempted robbery in 1999. Another boy shot the clerk, but because Jackson was present he was convicted of capital murder and aggravated robbery.
Miller was convicted of capital murder during the course of arson. A neighbor, while doing drugs and drinking with Miller and a 16-year-old boy, attacked Miller. Intoxicated, Miller and his friend beat the man and set fire to his home, killing the 52-year-old man. Miller's friend testified against him, and got life in prison with the possibility of parole.
"This is an important win for children. The court took a significant step forward by recognizing the fundamental unfairness of mandatory death-in-prison sentences that don't allow sentencers to consider the unique status of children and their potential for change," said Bryan Stevenson, executive director of the Equal Justice Initiative, who represented Jackson and Miller. "The court has recognized that children need additional attention and protection in the criminal justice system."
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. Seventy-nine of them are in prison for crimes that took place when they were 14 or younger. More than 2,000 of them were there because the sentence was mandated by a legislature.
Salt Lake Tribune reporter Janelle Stecklein contributed to this story.