WASHINGTON – Reversing its own 15-year-old precedent, the Supreme Court yesterday struck down the death penalty for prisoners who committed crimes when they were younger than 18.

A 5-4 majority of the court cited an evolving national and international consensus that such executions amount to cruelty. The ruling spares the life of a Missouri man who was 17 when he robbed and drowned an elderly woman, as well as more than 70 young people who are on death rows in 12 other states.

Writing for the majority, Justice Kennedy held that teenagers are too immature and vulnerable to be held to the same standards of culpability as adults.

“From a moral standpoint it would be misguided to equate the failings of a minor with those of an adult,” Justice Kennedy wrote in an opinion joined by Justices Stevens, Souter, Breyer, and Ginsburg.

The majority held that the execution of juveniles violates the Eighth Amendment’s ban on “cruel and unusual punishment.” Despite reaching the opposite conclusion in a 1989 case, the court said the meaning of the clause is subject to change over time in accordance with “the evolving standards of decency.”

In a scathing dissent joined by Chief Justice Rehnquist and Justice Thomas, Justice Scalia wrote that the decision was reached on “the flimsiest grounds.” He attacked the court for basing its decision not only on the evolving views of state legislatures, but also on the judges’ own determinations about whether the death penalty is appropriate for teenagers.

“The Court thus proclaims itself sole arbiter of our Nation’s moral standards,” wrote Justice Scalia.

Those three dissenters and Justice O’Connor found there is not a national consensus against the juvenile death penalty.

A former federal solicitor general, Seth Waxman, argued the case for Christopher Simmons, who was 17 when he robbed and later drowned a Missouri woman, Shirley Crook. Mr. Waxman said he was “delighted” that America has joined the rest of the world in outlawing the death penalty for juveniles.

But the president of a victims’ rights group that submitted a brief to the court called on state legislatures yesterday to defy what she called an “offensive” and “devastating” ruling.

“We need state legislatures to keep the statutes on the book. Let’s just defy them and stand up for states’ rights and for innocent people,” said the president of Justice for All, Dianne Clements.

“I am totally shocked and disappointed that our system has failed us the way it has,” said Janet Green, whose 25-year-old son Michael Eakin, a police officer, was killed in the line of duty by an assailant who turned 18 a few weeks after the crime.

Eakin’s killer cannot be executed under yesterday’s ruling.

The case was also watched closely around the world. Forty-eight countries filed written briefs with the court, arguing that the juvenile death penalty violated international norms.

Justice Kennedy noted “the stark reality” that America is the only country in the world that continues to give official sanction to the juvenile death penalty.

While the opinion of the world community was “not controlling” in the case, it offered significant and respected “confirmation” of the judges’ reasoning, he wrote.

Both parties in the case accepted data showing that only seven countries other than America have executed juvenile offenders since 1990: Iran, Pakistan, Saudi Arabia, Yemen, Nigeria, the Democratic Republic of Congo, and China. Each of these countries has since either abolished capital punishment for juveniles or publicly disavowed the practice.

“In sum, 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,” Justice Kennedy wrote.

He noted that the practice was banned by the United Nations Convention on the Rights of the Child, which has been ratified by every country save for America and Somalia.

But Justice Scalia accused the majority of substituting their own opinions and those of “likeminded foreigners” for law.

“What these foreign sources ‘affirm,’ rather than repudiate, is the Justices’ own notion of how the world ought to be, and their diktat that it shall be so henceforth in America,” Justice Scalia wrote.

If anything, America’s refusal to sign the U.N. convention should be evidence of the lack of a national consensus on the punishment, he wrote.

In addition to consideration of the relative immaturity of teenagers, Justice Kennedy’s majority opinion found that states had been turning against the punishment since the Supreme Court upheld the juvenile death penalty in a 1989 case called Stanford v. Kentucky.

Thirty states prohibit the execution of juveniles, including 12 that ban executions altogether, and 18 that exclude juveniles. In the 20 states that do not prohibit it, the execution of juveniles “is infrequent,” Justice Kennedy found.

In the past 10 years, only Oklahoma, Texas, and Virginia have executed prisoners for crimes committed as juveniles, he noted.

The majority’s decision was patterned on its 2002 decision in Atkins v. Virginia, in which the court ruled the death penalty unconstitutional when applied to defendants with mental retardation, because many states had banned the practice.

Justice Kennedy acknowledged that in contrast to mental retardation, there has been a “less dramatic” movement among states to abolish juvenile executions, but he wrote that the slower pace “is counterbalanced by the consistent direction of the change toward abolition.”

The dissenters objected to Justice Kennedy’s argument that justices should begin their reasoning by looking at state laws, but then use their own independent judgment to determine whether teenagers are sufficiently mature to be subject to the death penalty.

“I would not substitute our judgment about the moral propriety of capital punishment for 17-year-old murderers for the judgments of the Nation’s legislatures,” Justice O’Connor wrote in a separate dissent.

Giving a glimpse of her personal opinion on the matter, Justice O’Connor wrote, “Indeed, were my office that of a legislator, rather than a judge, then I, too, would be inclined to support legislation setting a minimum age of 18 in this context,” she wrote.

But for the purposes of the Constitution, the relevant differences between “adults” and “juveniles” appear to be “a matter of degree, rather than of kind.”

The dissenters also criticized the majority for not reprimanding the Missouri Supreme Court, which had single-handedly declared unconstitutional the juvenile death penalty in Simmons’s case, despite the Supreme Court’s 1989 precedent to the contrary.

“By affirming the lower court’s judgment without so much as a slap on the hand, today’s decision threatens to invite frequent and disruptive reassessments of our Eighth Amendment precedents,” Justice O’Connor said.

She also disagreed with Justice Scalia on the role of international law in the court’s reasoning under the Eighth Amendment, finding that for almost 50 years, the court has consistently referred to foreign and international law as relevant to its assessment of evolving standards of decency.

The decision was a victory for a wide array of groups that participated in the case, including youth groups, religious groups, psychologists, and medical professionals.

“It is a complete victory. The case does finally conclude this chapter in the American system of punishment,” said the legal director and co-founder of the Juvenile Law Center, Marsha Levick. The Philadelphia-based group had filed a friend-of-the-court brief supporting Simmons on behalf of 50 advocacy groups.

Ms. Levick predicted that the court’s recognition of developmental and culpability differences between teenagers and adults could “have some implications” for future sentencing of juvenile defendants.

Robert Grey, president of the American Bar Association, which also filed a brief, applauded the ruling as “truly a landmark decision.”

The attorney general of Missouri, Jay Nixon, said the state would respect the decision.

“There has never been any question about his guilt in the murder of Shirley Crook, and this decision confirms that he will spend the rest of his life in prison,” Mr. Nixon said in a statement.