A plea to declare capital punishment for juvenile offenders unconstitutional received a skeptical hearing at the Supreme Court yesterday, as one of two justices likely to be pivotal to the outcome repeatedly voiced doubts.

Justice Anthony M. Kennedy observed that he was "troubled" that a ban might lead gang leaders to use 16- or 17-year-olds as "hit men."

"I'm very concerned about that," Kennedy told Seth P. Waxman, a former U.S. solicitor general who was arguing on behalf of Missouri death row inmate Christopher Simmons, who faces execution for a murder he committed at 17. "I'm talking about the deterrent value of the existing rule."

Waxman replied that deterrence does not work with juvenile offenders because they "are impulsive and subject to peer pressure."

But Kennedy responded that many 17-year-old offenders were "ringleaders" in brutal, calculated murders, citing a friend-of-the-court brief filed by a group of states that wish to retain a death penalty for juveniles. Kennedy said the brief was "chilling reading."

Kennedy's vote is crucial because four justices, John Paul Stevens, David H. Souter, Ruth Bader Ginsburg and Stephen G. Breyer, have already declared their belief that executing those who commit crimes while under the age of 18 is "cruel and unusual punishment."

The records of Chief Justice William H. Rehnquist and Justices Antonin Scalia and Clarence Thomas suggest that they think the issue should be left up to the states.

That leaves only Kennedy and Justice Sandra Day O'Connor as possible fifth votes for a ban. O'Connor was mostly silent in yesterday's hearing.

The Supreme Court upheld capital punishment for 16- and 17-year-old offenders in 1989. It banned it for those 15 and under in 1988.

Until now, the court's position has been unaffected by its 2002 decision to ban the death penalty for the moderately mentally retarded. In that case, Kennedy and O'Connor joined Stevens, Souter, Ginsburg and Breyer.

The court found that a national consensus against the practice had formed since 1989. The best evidence for that, the court ruled, was that the number of states banning the death penalty for the retarded had grown from two to 13.

Many opponents of the death penalty immediately recognized that a similar argument could be applied to juveniles.

Yet in the 2002 opinion, written by Stevens, the court seemed to warn that it was not ready for that, contrasting the strong trend against executing the retarded with the fact that, between 1989 and 2002, only two states had raised their minimum ages for capital punishment to 18.

Three times in the past two years, the court refused to hear appeals from death row inmates who had killed while they were juveniles -- despite a strongly worded dissent in one case from Stevens, Souter, Ginsburg and Breyer.

Last year, the court actually reinstated the death penalty of a juvenile offender in Oklahoma after a federal appeals court had blocked it. The vote was 5 to 4, with Kennedy and O'Connor in the majority.

The court intervened in the Simmons case only after the Missouri Supreme Court threw out Simmons's sentence last year on the grounds that the 1989 precedent had, indeed, been superseded by the 2002 case.

Waxman pressed a similar argument yesterday. He told the court that the execution of juvenile criminals conflicts with a "robust consensus" reflected in the recent decisions of several states to ban the practice and in the decreasing frequency with which juries impose the death penalty on those under 18. Scientific evidence shows that the areas of the brain in charge of judgment and impulse control are incomplete in adolescents, he said.

Death is the wrong punishment for crimes that reflect "the transient psychosocial characteristics that rage in adolescents," Waxman said.

But Kennedy noted that the American Psychological Association -- which told the court in a brief in this case that adolescents are too immature to qualify for capital punishment -- had also said in a case about parental notification for abortion that teenagers are old enough to make such a decision on their own. "They flip-flopped," Kennedy said.

Waxman said there is no inconsistency because "what was at issue in that case was the competency to decide," while the question in this case involves "factors why adolescents are less morally culpable."

He urged the court to act based on a "worldwide consensus" against the death penalty for juveniles, noting that, except for the United States, all 110 countries that have capital punishment do not apply it to those under 18.

States that permit it "are not just alone in this country, they are alone in the world," he said.

On this point, Kennedy appeared to sympathize with Waxman, asking Missouri state Solicitor James R. Layton: "There seems to be a very substantial demonstration that world opinion is against us. . . . Does that have a bearing on what's 'unusual' " punishment?"

Layton replied that "what matters" is U.S. legislation.

O'Connor's only remark hinted at some openness to Waxman's position.

Addressing Layton, she noted that the number of states that either do not have capital punishment or set the age at 18 is "about the same" as the number of those with bans on executing the retarded in 2002, which was 30.

"There is no inexorable trend here," Layton replied.

The case is Roper v. Simmons, No. 03-633. A decision is expected by July.

Bill Pelke, standing in front of the Supreme Court, said he is opposed to the execution of killers who were juveniles when they committed their crimes.Christopher Simmons, 28, committed murder at 17.