CHESAPEAKE, Va., Dec. 24 — The decision by a jury here to spare Lee Malvo's life after finding him guilty in last year's Washington-area sniper rampage may hasten a movement to abolish the death penalty for juvenile killers, legal experts say.

The movement had stalled, they say, because Mr. Malvo's crimes, at age 17, were so troubling. Now, though, the jury's decision to let him live may prompt reconsideration of whether executing juvenile offenders is ever proper.

Though the death penalty for murders committed by 16- and 17-year-olds remains available in a minority of states, it is being imposed with less and less frequency. Public support for it is limited. It is essentially unknown in the rest of the world.

And the United States Supreme Court seems poised to reconsider two of its rulings, issued in 1988 and 1989, that together banned execution of those who were under 16 when they committed their crimes but allowed it for those who were 16 or over.

Several scholars say the Malvo case may have been a factor in the Supreme Court's recent decisions not to accept cases on the issue.

"Some of the speculation has been that they really didn't want to take the Malvo case out of the hands of the jury," Victor L. Streib, a law professor at Ohio Northern University who is an expert on the juvenile death penalty, said of the justices.

But four of those justices have indicated their discomfort with the execution of juvenile criminals.

One is Justice John Paul Stevens, who dissented last year from a court decision not to take up the constitutionality of the practice.

"Executing such offenders is a relic of the past," Justice Stevens wrote. "We should put an end to this shameful practice."

Justices Stephen G. Breyer, Ruth Bader Ginsburg and David H. Souter joined the dissent.

Even some death penalty supporters say it is only a matter of time before the Supreme Court bans the execution of juvenile criminals.

"It would have happened already were it not for Malvo," said Robert Blecker, a professor at New York Law School, who called himself a retributionist and added that such a decision would be constitutionally and morally suspect.

One opponent of the juvenile death penalty, Elizabeth S. Scott, a law professor at the University of Virginia, said the jury in the Malvo case had sent an important message.

"The Malvo verdict should be taken as a signal that the public has little enthusiasm for executing juveniles," Professor Scott said, "even for the most horrendous of crimes, and that people understand that young offenders are less culpable than adults."

As measured by opinion polls, public support for the death penalty itself remains strong. But it has dipped lately. A recent Gallup poll measured it at 64 percent, down six to eight percentage points from a year earlier and, says the Death Penalty Information Center, an opponent of capital punishment, the lowest level in 25 years.

Legal experts say possible explanations include public discomfort over a number of cases in which DNA and other evidence has exonerated death row prisoners, and the effect of Gov. George Ryan's mass commutation of 171 death sentences in Illinois on the ground that he could not be sure that innocent people were not among those condemned.

In any case, support for the juvenile death penalty is much weaker than for capital punishment over all; it ranged from 21 percent to 38 percent in recent polls.

The Missouri Supreme Court recently ruled that the execution of juvenile offenders was unconstitutional. That, according to the Death Penalty Information Center, made Missouri the 17th state to ban the practice among the 38 that allow capital punishment. Only one juvenile offender has been executed this year, Scott Hain in Oklahoma.

It was in 1988 that the Supreme Court prohibited the execution of defendants who were younger than 16 at the time of their crimes. The next year, the court declined to extend that ban to execution of offenders under 18.

The justices' decision in 2002 prohibiting the execution of the mentally retarded may provide a framework for their consideration of how juveniles should be treated. In the case on retardation, the court looked to trends in state legislatures, noting that 30 states prohibited the execution of retarded people, including states without the death penalty. The comparable number for juveniles is 29, not including the District of Columbia and the federal government.

Fifteen of the 21 states that allow the execution of juveniles have not carried out such an execution since the death penalty was reinstated in 1976; nine of them have no juveniles on death row.

The Malvo case may have cast a shadow over state legislatures' consideration of the issue.

"Some state legislators have said things like `We're not sure we want to do away with the juvenile death penalty, because we might have a Malvo case here,' " said Professor Streib, of Ohio Northern.

In the courts, the trend is toward fewer death sentences for juveniles.

"The annual death sentencing rate for juvenile offenses has been declining rapidly and is now at the lowest point in 15 years," Professor Streib wrote in a recent report. Two juvenile offenders have been sentenced to death in 2003.

Only three states have carried out executions of juvenile offenders with any frequency in recent decades. They are Texas, Virginia and Oklahoma, with 13, 3 and 2, which together account for more than 80 percent of the 22 juvenile executions in the United States since the death penalty was reinstated.

Professor Scott, opponent of juvenile executions, said science, too, lent evidence bolstering the case for abolishing them.

"Current knowledge about brain development in adolescence," she said, "supports the commonly held intuition that even 17-year-olds are less mature than adults, something that the law recognizes in almost every other area."

Richard C. Dieter, executive director of the Death Penalty Information Center, said American law was out of step with world opinion on the issue.

"The United States is even more isolated on the juvenile death penalty than on the death penalty in general," Mr. Dieter said. "We're one of two countries that have not ratified the Convention on the Rights of the Child," which prohibits such executions. The other is Somalia, he said.

In sum, Attorney General John Ashcroft's decision to send the Malvo case to Virginia may have turned out to be counterproductive.

"The ironic thing is that Attorney General Ashcroft's strategy in the case may end up backfiring," said Jamie Orenstein, a former Justice Department official who advised Attorney General Janet Reno on the death penalty. "By choosing to send Malvo's case to Virginia first precisely because of its juvenile death penalty law, he may end up hastening its demise."