Five justices, in an opinion by Justice Anthony M. Kennedy, agreed that the Eighth Amendment’s ban on cruel and unusual punishment forbids such sentences as a categorical matter.

“A state need not guarantee the offender eventual release,” Justice Kennedy wrote, “but if it imposes the sentence of life, it must provide him or her with some realistic opportunity to obtain release before the end of that term.”

The ruling marked the first time that the court excluded an entire class of offenders from a given form of punishment outside the context of the death penalty. “ ‘Death is different’ no longer,” Justice Clarence Thomas wrote in dissent.

The overall vote was 6-to-3, though that is a little misleading. Chief Justice John G. Roberts Jr. voted with the majority in saying that the inmate who brought the appeal had received a sentence so harsh that it violated the Constitution. But the chief justice endorsed only a case-by-case approach, saying that an offender’s age could be considered in deciding whether a life sentence was so disproportionate to the crime as to violate the Eighth Amendment.

The case involved Terrance Graham, who in 2003, at age 16, helped rob a Jacksonville restaurant, during which an accomplice beat the manager with a steel bar. Mr. Graham was sentenced to a year in jail and three years’ probation for that crime.

The next year, at 17, Mr. Graham and two 20-year-old accomplices committed a home invasion robbery. In 2005, a judge sentenced Mr. Graham to life for violating his probation.

The Supreme Court has carved out categories of offenders and crimes that are not subject to the death penalty, including juvenile offenders and those who do not take a life. Monday’s decision applied those two decisions to life-without-parole sentences.

Justice Kennedy, who was joined by Justices John Paul Stevens, Ruth Bader Ginsburg, Stephen G. Breyer and Sonia Sotomayor, said both national and international practices supported the court’s ruling.

Justice Thomas said the majority was wrong about the facts in the United States and abroad and wrong as a matter of principle to take account of international opinion. Justice Antonin Scalia joined all of Justice Thomas’s dissent and Justice Samuel A. Alito Jr. most of it.

Thirty-seven states, the District of Columbia and the federal government have laws allowing life-without-parole sentences for juveniles convicted of non-homicide offenses. That represents, Justice Thomas said, a super-majority of states in favor of the punishment.

Justice Kennedy responded that a study relied on by Mr. Graham and supplemented by the court’s own research had located only 129 juvenile offenders convicted under such laws. Seventy-seven were in Florida, the rest in 10 other states. Those numbers, Justice Kennedy said, make the sentence “exceedingly rare” and demonstrate that “a national consensus has developed against it.”

Justice Thomas drew a different conclusion from the same numbers. “That a punishment is rarely imposed demonstrates nothing more than a general consensus that it should be just that — rarely imposed,” he wrote. “It is not proof that the punishment is one the nation abhors.”

Justice Kennedy added that the sentences at issue had been “rejected the world over.” Indeed, only the United States and perhaps Israel, he said, impose the punishment even for homicides committed by juveniles.

“The judgment of the world’s nations that a particular sentencing practice is inconsistent with basic principles of decency,” Justice Kennedy wrote, “demonstrates that the court’s rationale has respected reasoning to support it.”

Justice Thomas disputed Justice Kennedy’s math, saying 11 nations seem to allow the punishment in theory. More important, he said, “foreign laws and sentencing practices” are “irrelevant to the meaning of our Constitution.”

He added that most democracies around the world remain free to adopt the punishment should they wish to. “Starting today,” Justice Thomas wrote, “ours can count itself among the few in which judicial decree prevents voters from making that choice.”

Although the majority limited its decision to non-homicide offenses, advocates may try to apply its logic more broadly to the some 2,000 inmates serving life-without-parole sentences in the United States for participating in killings at 17 or younger.

Justice Thomas questioned the distinction drawn by the majority between killings and other sorts of violent crimes. “The court is quite willing to accept that a 17-year-old who pulls the trigger on a firearm can demonstrate sufficient depravity and irredeemability to be denied re-entry into society,” he wrote, “but insists that a 17-year-old who rapes an 8-year-old and leaves her for dead does not.”

Justice Alito, in a separate dissent that seemed directed to sentencing judges, said the majority’s opinion did nothing to affect even quite long fixed sentences.

Justice Thomas predicted that Monday’s ruling would give rise to years of litigation about just what sort of parole or other processes states must provide to provide the required “meaningful opportunity to obtain release.”

The case decided Monday, Graham v. Florida, No. 08-7412, was argued in November along with a companion case, Sullivan v. Florida, No. 08-7621. The court declined to decide the second case, which involved Joe Sullivan, who raped a woman when he was 13.

Instead, the court dismissed the case as improvidently granted, probably because it was beset with procedural difficulties. Mr. Sullivan’s lawyer, Bryan Stevenson, said his client and everyone else in his situation would be entitled to challenge their sentences under the Graham decision.

As usual in cases involving the Eighth Amendment, the justices debated whether the Constitution should consider, in a one common formulation, “the evolving standards of decency that mark the progress of a maturing society.”

Justice Thomas said the court should look to the practices at the time the Bill of Rights was adopted. Given that capital punishment could be imposed on people as young as 7 in the 18th century, he said, Mr. Graham’s punishment would almost certainly have been deemed acceptable back then.

Justice John Paul Stevens, in a concurrence joined by Justices Ginsburg and Sotomayor, said Justice Thomas’s “static approach to the law” did not allow for societal progress and would entail unacceptable human consequences.

“Justice Thomas would apparently not rule out a death sentence for a $50 theft by a 7-year-old,” Justice Stevens wrote. “Knowledge accumulates,” he wrote. “We learn, sometimes, from our mistakes.” ...