The Supreme Court Thursday agreed to consider banning the death penalty for all juvenile murderers, a day after issuing a decision that could end capital punishment for killers not yet 16 when they committed their crimes.

The justices said they will decide sometime next year whether use of the death penalty for anyone under 18 when the crime was committed violates the Constitution's ban on "cruel and unusual punishment."The court voted to consider overturning the death sentences of Georgia death row inmate Jose Martinez High, who was 17 when he participated in a 1976 murder, and Missouri death row inmate Heath Wilkins, who was 16 when he commited a 1985 murder.

The court Wednesday overturned the death sentence of an Oklahoma man convicted of a murder committed when he was 15.

Four of the eight justices who voted in that case said the death penalty for convicted murderers who committed their crimes before reaching 16 always amounts to unlawfully "cruel and unusual punishment."

Three justices said no such age limitation on imposing capital punishment can be derived from the Constitution.

Justice Anthony M. Kennedy did not participate in Wednesday's ruling but will vote in the two cases to be decided next year.

In her controlling vote Wednesday, Justice Sandra Day O'Connor voiced serious doubts whether states may execute people for crimes committed when only 15.

"Although I believe that a national consensus forbidding the execution of any person for a crime committed before the age of 16 very likely does exist, I am reluctant to adopt this conclusion as a matter of constitutional law without better evidence than we now possess,"' O'Connor said.

But she supplied a fifth vote to overturn William Wayne Thompson's death sentence, ruling that the state's death penalty law could not be applied to him because it has no minimum age.

Wednesday's ruling left unclear whether O'Connor would vote to strike down a death penalty law that explicitly made 15-year-old murderers eligible for execution.

The decision also left unclear whether the court would find fault with death penalty laws in which convicted killers who were 16 or 17 when they committed their crimes are eligible for execution.

Those questions could be answered when the court completes its study, expected to begin when the court starts its new term in October, of the appeals filed in behalf of High and Wilkins.

High was sentenced to death for killing an 11-year-old boy during a gas station robbery in Taliferro County, Ga.

He was convicted in the July 26, 1976, shooting death of Bonnie Bulloch, the step-son of a gas station attendant.

High was weeks short of his 18th birthday when the crime was committed.

Wilkins was 16 when on July 27, 1985, he helped rob an Avondale, Mo., liquor store.

Nancy Allen, a clerk in the store, was fatally stabbed during the robbery.

Lawyers around the country still were absorbing the impact of Wednesday's ruling on the death penalty for young offenders.

"As a practical matter, l5-year-olds will not be executed in this country," predicted Paul L. Hoffman, a Los Angeles lawyer for Amnesty International. "Justice O'Connor walked right up to the edge of saying that."

In Oklahoma, Assistant Attorney General David Lee said: "The line has been drawn in my opinion. Our reading of the opinion is that (the court) used 16 as the line."

If Hoffman and Lee are correct, the decision will affect only three of the some 2,100 people now on death row nationwide. But Thompson's case nevertheless had captured worldwide attention.

Of the 100 men and women executed in U.S. prisons since the Supreme Court in 1976 allowed states to resume capital punishment, three died for murders committed when they were 17.

Since 1948, no one has been executed in this country for a crime committed while under the age of 17.

Of the 37 states with death penalty laws, 19 of them do not specify a minimum age. Of the 18 state laws tht do impose a minimum age, none of them allows execution of killers younger than 16 when they committed their crimes.

Other actions

In other action, the court:

Apparently removed the last legal obstacle to a $180 million settlement between Vietnam veterans and manufacturers of the herbicide Agent Orange.

The justices, without comment, let stand rulings that dismissed lawsuits by about 300 veterans who challenged the settlement, in which some 120,000 others are expected to participate.

- Agreed to study alleged race discrimination in Alaskan salmon canneries, setting the stage for an important sequel to a one-day-old ruling that makes it easier for workers to prove their bosses are guilty of illegal bias.

- Agreed to hear the appeal of a Texas man sentenced to death who, the justices were told, has the reasoning capacity of a 7-year-old.

- Agreed to decide whether states may ban virtually all lawyer advertisements featuring testimonials or client endorsements.

- Rejected the appeal of a Minnesota woman severely injured in a 1983 car crash, turning away arguments that her car's manufacturer negligently failed to install airbags.

- Refused to kill a libel lawsuit against a Tennessee congressman who criticized a legal services lawyer.