WASHINGTON, Oct. 13 - If American society has indeed reached a consensus that the death penalty should not apply to those who kill at age 16 or 17, as the lawyer for a young Missouri murderer argued to the Supreme Court on Wednesday, no such consensus was apparent among the justices themselves.

Two years after ruling 6 to 3 that the execution of mentally retarded offenders is categorically unconstitutional, the court appeared deeply divided over whether the reasoning of that decision meant that the death penalty for acts committed while a juvenile should likewise be seen as "cruel and unusual punishment" in violation of the Eighth Amendment.

The Missouri Supreme Court reached that conclusion by a 4-to-3 decision in August 2003, freeing Christopher Simmons from death row for a murder he committed in 1993 when he was 17. It resentenced him to life in prison without parole.

Missouri appealed to the United States Supreme Court, arguing that the state court lacked authority to reject the Supreme Court's last decision on the question, a 1989 ruling that upheld capital punishment for 16- and 17-year-olds. A 1988 decision barred the execution of those who killed when they were younger than 16.

Seth P. Waxman, representing Mr. Simmons, argued that not only the increasing rarity of juvenile executions since 1989 but also new medical and psychological understanding of teenage immaturity validated the step the Missouri court took last year.

"These developments change the constitutional calculus," Mr. Waxman, a former United States solicitor general, told the justices. The new scientific evidence, described in briefs filed by the American Medical Association, the American Psychological Association and other professional groups, "explains and validates the consensus that society has drawn," he said.

Justice Antonin Scalia countered: "If all this is so clear, why can't the legislature take it into account? All you have to do is bring these facts to the attention of the legislature."

Mr. Waxman replied that the number of states that actually execute people for crimes committed as juveniles is "very small." While 19 states nominally permit the execution of 17-year-old murderers, only three states - Texas, Virginia and Oklahoma - have executed juvenile offenders in the past 10 years.

Oklahoma has no juvenile offender on its death row. Virginia has one, and a jury there refused last year to impose a death sentence after finding Lee Malvo, the teenage member of a pair of Washington-area snipers, guilty of murder. Texas, with 29 inmates now on death row for juvenile crimes, accounts for more than half the executions of juvenile offenders, 13 of 22, carried out in the United States since the modern era of capital punishment began in 1976. There were 2 juvenile death sentences imposed in the United States last year and 1 so far this year, down from 14 five years ago.

Justice Scalia told Mr. Waxman he was not surprised by the low numbers. They demonstrated juries' ability to take a defendant's youth into consideration, he said, adding that the question was whether to leave it to juries or to impose a "hard rule."

Chief Justice William H. Rehnquist challenged Mr. Waxman on whether the scientific evidence contained in the briefs was even appropriate for the court's consideration. Noting that the studies had not been introduced at Mr. Simmons's trial, he said, "You're talking facts, and facts are ordinarily adduced at trial for cross-examination."

Mr. Waxman, temporarily nonplussed, replied: "The issue for this court is not the application of law to a particular defendant, but what the Constitution requires as a matter of law."

Justice Anthony M. Kennedy asked Mr. Waxman whether he would lose the case if the court accepted neither the scientific evidence nor the existence of a consensus.

"This is truly a case in which the whole is greater than the sum of the parts," Mr. Waxman replied.

Four justices - John Paul Stevens, David H. Souter, Ruth Bader Ginsburg and Stephen G. Breyer - have made clear in recent years their desire to invalidate the juvenile death penalty. "The practice of executing such offenders is a relic of the past and is inconsistent with evolving standards of decency in a civilized society," the four wrote in October 2002, dissenting from the court's refusal to grant a writ of habeas corpus to a Kentucky inmate, an action that required five votes.

Just as clearly, Chief Justice Rehnquist and Justices Scalia and Clarence Thomas, the three dissenters in the retardation case, will not vote to extend that decision to juveniles.

With these facts known to most people in the courtroom, the focus of attention was on Justice Kennedy and Justice Sandra Day O'Connor, both of whom rejected the challenge to juvenile executions in 1989 and at least one of whom must repudiate that precedent if Mr. Simmons is to prevail.

Justice O'Connor, usually an active participant in the court's arguments, made only one comment, to James R. Layton, Missouri's state solicitor. She noted that the number of states that have rejected execution of those younger than 18 was "about the same" as the number that had rejected execution of the retarded in the years leading up to the court's ruling in that case. Of the 38 states with a death penalty, 19 have a minimum age of 18. In 2002, 18 states barred execution of the retarded. "Are we at least required to look at that?" Justice O'Connor asked.

Mr. Layton replied that the retardation case, Atkins v. Virginia, took account of an "inexorable trend" among the states, and "we don't have that here." In the retardation case, there had been what the court called a "dramatic shift in the state legislative landscape," with only two states having barred execution of the retarded as recently as 1989.

In fact, Justice Stevens, in his majority opinion in the retardation case, went out of his way in a footnote to contrast that shift with the much slower rate of change on the youth question. The footnote may have been necessary to hold the vote of Justice O'Connor or Justice Kennedy.

On Wednesday, Justice Kennedy appeared deeply conflicted throughout the argument. He said he was concerned that drawing the line at 18 might induce teenage gangs to designate their 16- or 17-year-old members as "hit men." A brief filed by Alabama that contained grisly descriptions of murders committed by teenagers made for "chilling reading," Justice Kennedy said, adding that he wished all those who had signed briefs for Mr. Simmons "had read it before they signed on." This led Justice Stevens to say that the death penalty did not seem to have deterred those crimes, all of which took place in states that permit the execution of juvenile offenders.

The case, Roper v. Simmons, No. 03-633, has attracted wide interest overseas, with briefs for Mr. Simmons signed by the European Union, the 45-member Council of Europe, and other organizations. The United States and Somalia are the only nations that have not formally repudiated executing juveniles. A brief filed by former United States diplomats asserted that the situation was an irritant in international relations.

Should the court give that brief any credence, Justice Stevens asked Mr. Layton. No, Missouri's lawyer replied, the question remained one for legislatures and not courts.