The ruling ends a practice used in 19 states and tosses out death sentences of about 70 juvenile murderers. It also blocks states from seeking to execute minors for future crimes.
"This is by far the biggest decision of the term," says CBSNews.com Legal Analyst Andrew Cohen. "It's a for opponents of the death penalty and a major defeat for many prosecutors around the country."
The court had already barred the death penalty for defendants 15 years of age or younger at the time of their crimes. Tuesday's ruling prevents states from making 16- and 17-year-olds eligible for execution.
The ruling had an immediate benefit for , convicted of the Washington D.C. snipings of 2002. Malvo was 17 at the time.
The decision was the second major defeat at the high court in three years for supporters of the death penalty. Justices in 2002 banned the execution of the mentally retarded, also citing the Constitution's Eighth Amendment ban on cruel and unusual punishments.
Justice Anthony Kennedy, writing for the majority, noted that most states don't allow the execution of juvenile killers and those that do use the penalty infrequently. The trend, he noted, was to abolish the practice.
"Our society views juveniles ... as categorically less culpable than the average criminal," Kennedy wrote.
Juvenile offenders have been put to death in recent years in just a few other countries, including Iran, Pakistan, China and Saudi Arabia. All those countries have gone on record as opposing capital punishment for minors.
The Supreme Court has permitted states to impose capital punishment since 1976 and more than 3,400 inmates await execution in the 38 states that allow death sentences.
Justices were called on to draw an age line in death cases after Missouri's highest court overturned the death sentence given to a 17-year-old Christopher Simmons, who kidnapped a neighbor in Missouri, hog-tied her and threw her off a bridge. Prosecutors say he planned the burglary and killing of Shirley Crook in 1993 and bragged that he could get away with it because of his age.
The four most liberal justices had already gone on record in 2002, calling it "shameful" to execute juvenile killers. Those four, joined by Kennedy, also agreed with Tuesday's decision: Justices John Paul Stevens, David H. Souter, Ruth Bader Ginsburg and Stephen Breyer.
Chief Justice William H. Rehnquist and Justices Antonin Scalia and Clarence Thomas, as expected, voted to uphold the executions. They were joined by Justice Sandra Day O'Connor.
Currently, 19 states allow executions for people under age 18: Alabama, Arizona, Arkansas, Delaware, Florida, Georgia, Idaho, Kentucky, Louisiana, Mississippi, Nevada, New Hampshire, North Carolina, Oklahoma, Pennsylvania, South Carolina, Utah, Texas and Virginia.
In a dissent, Scalia decried the decision, arguing that there has been no clear trend of declining juvenile executions to justify a growing consensus against the practice.
"The court says in so many words that what our people's laws say about the issue does not, in the last analysis, matter: 'In the end our own judgment will be brought to bear on the question of the acceptability of the death penalty,' he wrote in a 24-page dissent.
"The court thus proclaims itself sole arbiter of our nation's moral standards," Scalia wrote.