ASHINGTON, Jan. 26 — The Supreme Court agreed Monday to decide whether the Constitution prohibits the death penalty for crimes committed at the age of 16 or 17.

With capital punishment for juvenile offenders falling rapidly into disuse across most of the country — only two such death sentences were imposed last year and jurors in Virginia rejected the death penalty for Lee Malvo, the teenage sniper whom they convicted of murder — the justices will consider whether there is now a national consensus of the sort the court discerned two years ago when it prohibited the execution of mentally retarded defendants.

The outcome is uncertain. Opponents of the death penalty have pressed the court for several years to reconsider a 1989 decision that upheld capital punishment for older teenagers; a decision in 1988 struck down the death penalty for those age 15 and younger.

Four justices — John Paul Stevens, David H. Souter, Ruth Bader Ginsburg, and Stephen G. Breyer — have made clear their opposition to the death penalty for 16- and 17-year-olds, calling it "inconsistent with evolving standards of decency in a civilized society" when they dissented in 2002 from the court's refusal to take up the issue.

Despite that strong language, the four conspicuously did not manage to attract a fifth vote for their position then, and it is far from certain that a fifth vote exists today. In fact, the impetus to take up the issue this time may well have come from the court's more conservative members, responding to an appeal filed by the state of Missouri from a ruling by the Missouri Supreme Court last August that overturned the death sentence of a man who killed at age 17.

The defendant, Christopher Simmons, who is now 27, was convicted in 1993 of murdering a woman who recognized him when he broke into her house with a 15-year-old companion to commit a burglary.

A majority in the Missouri Supreme Court's 4-to-3 decision relied heavily on the United States Supreme Court's analysis in the mental retardation case, Atkins v. Virginia, for their conclusion that the execution of those who were younger than 18 when they committed their crimes had become so rare as to be "cruel and unusual punishment," prohibited by the Eighth Amendment.

Only three states, Texas, Virginia, and Oklahoma, had executed juvenile offenders in the past 10 years, the state court observed.

Jeremiah W. Nixon, the Missouri attorney general, said in his appeal that the state court had defied a directly relevant precedent of the United States Supreme Court, the 1989 ruling in Stanford v. Kentucky.

"The decision in Stanford has not been overturned by this court, even though the court has had recent opportunities to do so," Mr. Nixon told the justices. He said the state court's departure threatened to "wreak havoc throughout the justice system."

The Stanford ruling held that there was no national consensus on executing juveniles and let stand the death penalty imposed on the defendant, Kevin N. Stanford. Mr. Stanford's sentence was later commuted by the governor of Kentucky.

The court will not hear the Missouri case, Roper v. Simmons, No. 03-633, until next October. Because the decision will turn on the court's view of the existence of a national consensus, the briefs of each side are likely to display dueling interpretations of the same evidence.

Of the 38 states that have a death penalty, 17 have set a minimum age of 18, either by legislation or judicial decision. That is an increase of five states since the Supreme Court found a lack of consensus on the question in its 1989 decision.

By contrast, only two states that had a death penalty barred execution of the mentally retarded in 1989, when the Supreme Court in a separate decision rejected a challenge to capital punishment for that category of defendants. The number grew to 18 by the time the court, concluding that there had been a "dramatic shift in the state legislative landscape," ruled in the Atkins case in 2002.

Justice Stevens, in a footnote to his majority opinion in the Atkins case, compared the trends in punishing mentally retarded and juvenile defendants, finding no such dramatic shift on the juvenile question. The contrast "is telling," Justice Stevens said.

Since his own view was that the death penalty for juveniles was unconstitutional, it is highly likely that he inserted that footnote to assure Justices Anthony M. Kennedy and Sandra Day O'Connor, both of whom joined the 6-to-3 majority, that the retardation decision did not foreordain a similar outcome for juveniles.

But the decision did not foreclose taking the next step. In retrospect, the footnote appears simply to have underscored the obvious: that Justices O'Connor and Kennedy hold the balance of power on the juvenile death penalty. Death penalty opponents need to persuade only one of them to prevail.

To that end, the opponents are likely to stress not only growing domestic opposition to executing juveniles but also, from an international perspective, the unmistakable isolation of the United States. In speeches and opinions, both Justices O'Connor and Kennedy have expressed growing interest in international legal developments.

The United States is the only country in which the execution of those under 18 is officially sanctioned and the only country that has not signed the United Nations Convention on the Rights of the Child, which prohibits the practice. Worldwide, the last five executions of juvenile offenders have taken place in the United States, the most recent in Oklahoma last April.

Earlier this month, Amnesty International began a campaign to abolish what it calls the "heinous practice" of executing juvenile offenders.

The practice in the United States is highly uneven. Texas alone has accounted for 13 of the 22 juvenile executions since the modern era of capital punishment began in 1976. Of the 73 people currently on death row for crimes committed before the age of 18, almost a third are in Texas.

Since executing Scott A. Hain last year, Oklahoma has had no juvenile offenders on its death row. It has not sentenced any to death in the past eight years. Nationwide, only two juveniles were sentenced to death in 2003, the fewest in 15 years.

The juvenile death penalty issue came under public scrutiny last year when the Bush administration arranged to have Mr. Malvo, the younger of the pair of Washington-area snipers, tried in Virginia so that he might face the death penalty for a murder he was charged with committing at age 17. The jury convicted him of the murder but voted to spare his life.

Impact on Texas

HUNTSVILLE, Tex., Jan. 26 (AP) — Twenty-six Texas death row inmates, including three scheduled for lethal injection in the coming months, would be affected if the Supreme Court barred execution of convicted killers whose crimes were committed when they were younger than 18.

Under Texas law, a person who commits a capital murder at 17 can be sentenced to death. There are other states that extend capital punishment to 16-year-olds, but in Texas a 16-year-old charged with capital murder receives an automatic life prison term if convicted.

About a dozen men have been executed in Texas for crimes committed when they were 17 since 1982, when the state resumed executions. The three now on the execution schedule, all Harris County cases, are : Eddie Capetillo, set to die March 30; Efrain Perez, June 23; and Raul Villarreal, June 24.

Roe Wilson, who handles capital case appeals for the Harris County district attorney's office, said Mr. Capetillo's lawyer was already inquiring about seeking a reprieve.

Of the 26 now on death row in Texas, where the total death row population is 451, 10 are from Harris County. The most recent inmate to be executed for a crime committed when he was 17 was Toronto Patterson, who was 24 when he was put to death in August 2002 for killing a 3-year-old cousin at her Dallas home.