ASHINGTON, March 1 - Concluding that the United States and the world have turned against the death penalty for youthful offenders, the Supreme Court ruled on Tuesday that the Constitution categorically bars capital punishment for crimes committed before the age of 18.

The 5-to-4 decision, which upheld a ruling by the Missouri Supreme Court, will move 72 people off death row in 12 states. It represented an about-face for a court that only 16 years ago rejected the argument that the execution of those who kill at the age of 16 or 17 violated the Eighth Amendment's prohibition against "cruel and unusual punishments."

Writing for the court on Tuesday, Justice Anthony M. Kennedy, who voted with the majority 16 years ago, said the new decision was necessary to keep pace with the "evolving standards of decency" that for the last 50 years have shaped the Supreme Court's view of what constitutes cruel and unusual punishments.

Justice Kennedy said that not only did 30 states - five more than 16 years ago - now reject the death penalty for juveniles, but that "it is fair to say that the United States now stands alone in a world that has turned its face against the juvenile death penalty."

Since 1990, he noted, only seven countries outside the United States have executed people for crimes they committed as juveniles, and all seven - Iran, Pakistan, Saudi Arabia, Yemen, Nigeria, China and Congo - have disavowed the practice.

There have been 19 such executions in the United States since 1990, most recently in 2003. Once the Supreme Court agreed in January of last year to decide the issue, all executions that stood to be affected by the decision were put on hold.

Justice Antonin Scalia, author of the court's last ruling on the subject, Stanford v. Kentucky, filed a dissent on the new decision in Roper v. Simmons, No. 03-633. Joined by Chief Justice William H. Rehnquist and Justice Clarence Thomas, Justice Scalia disputed nearly all of the majority's points, from how to count the various states to whether reference to the views of foreign legal systems had relevance or legitimacy.

"I do not believe that the meaning of our Eighth Amendment, any more than the meaning of other provisions of our Constitution, should be determined by the subjective views of five members of this court and like-minded foreigners," Justice Scalia said.

Justice Sandra Day O'Connor wrote a separate dissenting opinion that took issue both with the majority's assessment of the existence of a national consensus and with its view that the execution of juveniles was always disproportionate to their moral culpability. She noted that while as a legislator she would be "inclined to support" a minimum age of 18 for capital punishment, she could not find a constitutional basis for doing so as a judge.

Justice O'Connor pointedly disagreed, however, with her fellow dissenters' opposition to taking into account foreign legal developments. "This nation's evolving understanding of human dignity certainly is neither wholly isolated from, nor inherently at odds with, the values prevailing in other countries," she said.

Justice Kennedy's majority opinion was joined by Justices John Paul Stevens, David H. Souter, Ruth Bader Ginsburg and Stephen G. Breyer. Justices Stevens and Ginsburg filed a concurring opinion to say that "perhaps even more important than our specific holding today is our reaffirmation of the basic principle that informs the court's interpretation of the Eighth Amendment"; namely, that the amendment's meaning was not frozen at the time of its adoption but has evolved.

That evolution has been on lively display in recent years with respect to the death penalty. In 1988, in Thompson v. Oklahoma, the court barred the execution of those under 16 at the time of the crime. Three years ago, in Atkins v. Virginia, the court ruled that the Eighth Amendment categorically barred the execution of the mentally retarded. The contrary precedent that the Atkins decision overruled was handed down on the same day in 1989 as the ruling that the court repudiated on Tuesday, permitting the execution of those who killed at 16 or 17.

The court based the Atkins decision on its perception that a new consensus had formed in the country to reject the execution of the retarded; 16 states that had permitted such executions in 1989 had rejected them by the time the issue came back to the Supreme Court in 2000. After that decision, the attention of opponents of the death penalty immediately turned to the juvenile question, on which the development in the states since 1989 had been slower, but steady and uniformly in the direction of raising the age.

The defendant in the new case, Christopher Simmons, who was sentenced to death for a murder he committed in 1993 at 17, went back to the Missouri courts after the Atkins decision to argue that the reasoning of the retardation case should be applied to juvenile offenders.

The Missouri Supreme Court agreed, noting that five states since 1989, by legislation or judicial decision, had raised the age to 18. It ordered Mr. Simmons re-sentenced to life in prison without parole.

In their dissenting opinions on Tuesday, both Justice O'Connor and Justice Scalia were highly critical of the Missouri court, saying that it lacked authority to depart, on its own, from the Supreme Court's binding precedent. "Allowing lower courts to reinterpret the Eighth Amendment whenever they decide enough time has passed for a new snapshot leaves this court's decisions without any force," Justice Scalia said, adding, "The result will be to crown arbitrariness with chaos."

The majority did not engage in this particular debate. Instead, Justice Kennedy took the Missouri Supreme Court's opinion as a given and then proceeded through his own analysis of the factual and legal landscape.

That analysis had three parts. First was an assessment of whether American society had formed a consensus against juvenile executions. While the change was "less dramatic" than the change on the retardation question, Justice Kennedy said, it nonetheless provided the "objective indicia of consensus" that the court found to be enough in the retardation case. He noted that Congress had set a minimum age of 18 when it passed the Federal Death Penalty Act in 1994. Counting the 12 states with no death penalty, he said, a majority of the states have rejected capital punishment for juvenile crimes.

In his dissent, Justice Scalia said it made little sense to include the non-death-penalty states in this calculation, calling the majority's approach "rather like including old-order Amishmen in a consumer-preference poll on the electric car." He continued: "Of course they don't like it, but that sheds no light whatever on the point at issue." Without counting the non-death-penalty states, he said, only 18 states had legislatively rejected the execution of 16- and 17-year-olds.

The second step in the majority's analysis was to move beyond counting by states to decide, in Justice Kennedy's words, "in the exercise of our own independent judgment, whether the death penalty is a disproportionate punishment for juveniles."

He said that "three general differences between juveniles under 18 and adults demonstrate that juvenile offenders cannot with reliability be classified among the worst offenders." The differences were: "a lack of maturity and an underdeveloped sense of responsibility"; vulnerability to peer pressure; and a personality that is still in formation, making it "less supportable to conclude that even a heinous crime committed by a juvenile is evidence of irretrievably depraved character."

Justice Kennedy concluded: "When a juvenile offender commits a heinous crime, the state can exact forfeiture of some of the most basic liberties, but the state cannot extinguish his life and his potential to attain a mature understanding of his own humanity."

This analysis, too, provoked Justice Scalia's strong dissent. "By what conceivable warrant can nine lawyers presume to be the authoritative conscience of the nation?" he asked. After describing details of Mr. Simmons's crime, which involved breaking into a woman's home, binding her with tape and wire and throwing her off a bridge into a river, where she drowned, Justice Scalia said the majority had failed to justify "a constitutional imperative that prevents legislatures and juries from treating exceptional cases in an exceptional way by determining that some murders are not just the acts of happy-go-lucky teenagers, but heinous crimes deserving of death."

Justice Scalia reserved his strongest dissent for the final stage of the majority's analysis, its reference to international developments that have left the United States alone in supporting juvenile executions. In the majority opinion, Justice Kennedy said that while the court was not bound by foreign developments, "it is proper that we acknowledge the overwhelming weight of international opinion" for its "respected and significant confirmation for our own conclusions."

Justice Scalia objected that while the court had determined that "the views of our own citizens are essentially irrelevant," it had wrongly given "center stage" to the "so-called international community."