WASHINGTON – The justices of the Supreme Court debated yesterday whether to end the death penalty for murderers who were under 18 when they took a life, in a case that has drawn intense scrutiny from governments across the world.

Remarks by Justice O’Connor at an oral hearing yesterday hinted she may be prepared to add a crucial fifth vote to the four judges who have been on record for two years stating that the death penalty for 16- and 17-year-olds should be struck down.

Justice O’Connor said a national consensus among state lawmakers and juries against the execution of offenders under 18 is similar to one the court found against the execution of people with mental retardation when it struck down that practice in 2002, after having upheld it 13 years before.

“It’s about the same consensus that existed in [the case of] mental retardation,” she told the state solicitor of Missouri, James Layton. “Aren’t we required to look at that?”

Justice O’Connor provided the key vote that struck down the death penalty for offenders under the age of 16 in 1988.

Missouri is seeking the execution of Christopher Simmons, who was 17 when he abducted and threw into a river a woman whose home he had broken into and robbed.

The Missouri Supreme Court said his death sentence violated the Eight Amendment’s prohibition against cruel and unusual punishment, basing its ruling on the top court’s reasoning in the mental retardation case.

Mr. Layton argued that the age of 18 is “essentially an arbitrary line” that should be left to state lawmakers to determine based on scientific facts. In contrast to mental retardation, age is a mere “proxy” for maturity, and one factor among many that juries can consider when deciding the culpability of a defendant, he said.

Justice Ginsburg said 18 is a “pervasive” dividing line between youth and adulthood in America society, limiting the right to use tobacco, vote, sit on juries, and serve in the military.

She was one of four justices, including Justices Stevens, Souter, and Breyer, who wrote in 2002 that the death penalty is unconstitutional for offenders under 18.

Simmons’s lawyer, the former solicitor general in the Clinton administration, Seth Waxman, argued that an adolescent’s character is “not hard-wired,” and that a jury sitting two years after the crime cannot say for sure that the crime committed by a 17-year-old proceeded from an “enduring quality” of his character, or some “transient” aspect of his teenage years.

Mr. Waxman also argued that America stands “alone in the world” in allowing the practice.

The case has drawn exceptional international interest, with 48 countries signing a written brief urging the court to strike down the practice as contrary to international law and norms. The court also received briefs from a group of former American diplomats and recipients of the Nobel Peace Prize.

As if to underscore the international scrutiny, the Chief of Justice of the Supreme People’s Court of China, Xiao Yang, and several other judges from China listened to the oral argument with the help of simultaneous translation over a wireless headset.

Their visit had been planned in advance and had not been scheduled intentionally to coincide with the hearing, said a spokeswoman for the court.

But Mr. Waxman noted that China, along with Saudi Arabia, Nigeria, and the Democratic Republic of Congo, had recently banned the practice.

While Justice Breyer said world opinion is “relevant but not controlling” to the case, Mr. Layton argued that world opinion was no more relevant to the issue than if America was the only country that banned the juvenile death penalty.

Justice Scalia said countries disagree on other areas of criminal law. “Almost all of them don’t have trial by jury,” he said, and asked whether America should “yield” on that point as well.

Mr. Waxman argued that even the framers of the Constitution were concerned about the laws and opinions of other countries.

Justice Ginsburg agreed, saying of Thomas Jefferson, “Did he not say we had to show a decent respect for the opinions of mankind?” in a reference to the opening paragraph of the Declaration of Independence.

Seventy-four people who remain on death row could have their sentences vacated if Simmons wins, according to the Death Penalty Information Center. A ruling in his favor would also bolster the broader movement for abolition of the death penalty.

In 1989, 11 states barred the execution of people under 18. Since then 12 states have abolished the death penalty altogether. Four more have legislated against the juvenile death penalty. New York and Kansas have added the death penalty for first-degree murder, but their definition of the crime excluded offenders under the age of 18.Washing ton State’s juvenile death penalty was struck down by a state court. Out of the 38 states that retain the death penalty, 18 do not execute juveniles.

Mr. Layton said the cases of New York, Kansas, and Washington should not be interpreted as reflecting a consensus against the juvenile death penalty. However, New York and Kansas told the court in a written brief that they oppose the punishment.

Justice Stevens noted that the number of juvenile executions has fallen from 13 a decade ago to two last year. But Mr. Layton said such numbers do not account for variations in the number of youths committing capital crimes.

If the judges strike down a penalty that voters in his state believe to be necessary for the deterrence and retribution of heinous crimes, they will be “leading” rather than “reflecting society’s norms,” he warned.

Another potential swing voter, Justice Kennedy, expressed reservations about striking down the penalty, saying he was “very concerned” that excluding juveniles from he harshest punishment would make them more likely than adults to be persuaded to be the “hit men” in criminal gangs.

Mr. Waxman responded that the possibility of a death sentence had little deterrent value in Simmons’s case because he reportedly told his younger accomplices that they could “get away with it” because they were not adults.

Mr. Layton conceded that there is inconclusive evidence about whether the death penalty deters crime, but precisely because the facts are “inconclusive,” he said the issue should be left to legislatures to decide.