A closely divided US Supreme Court outlawed the death penalty for juvenile criminals, writing of a national consensus that such executions were unconstitutionally cruel.
Tuesday's ruling ended a practice that had brought international condemnation.
The 5-4 decision, which overturns a 1989 high court ruling, throws out the death sentences of 72 murderers who committed their crimes as juveniles and bars states from seeking to execute others. Nineteen states had allowed death sentences for people who killed when they were under 18.
The ruling was greeted with enthusiasm by numerous death penalty opponents in the US and abroad.
Justice Anthony Kennedy, writing for the majority, said many juveniles lack sufficient maturity and intellectual development to understand ramifications of their actions.
"The age of 18 is the point where society draws the line for many purposes between childhood and adulthood. It is, we conclude, the age at which the line for death eligibility ought to rest," Kennedy said.
The US has stood almost alone in the world in officially sanctioning juvenile executions, a "stark reality" that can't be ignored, Kennedy wrote. Juvenile offenders have been put to death in recent years in only a few other countries, including Iran, Pakistan, China and Saudi Arabia.
"It is proper that we acknowledge the overwhelming weight of international opinion against the juvenile death penalty, resting in large part on the understanding that the instability and emotional imbalance of young people may often be a factor in the crime," Kennedy wrote.
In an angry dissent, Justice Antonin Scalia disputed that a "national consensus" exists and said the majority opinion was based on the "flimsiest of grounds." The appropriateness of capital punishment should be determined by individual states, not "the subjective views of five members of this court and like-minded foreigners," he wrote.
The ruling continues the court's practice of narrowing the scope of the death penalty, which it reinstated as permissible punishment in 1976. Execution of those 15 and younger when they committed their crimes was outlawed in 1988. Three years ago, justices banned execution of the mentally retarded, citing a "national consensus" against putting to death a killer who may lack the intelligence to fully understand his crime.
In finding a similar consensus against juvenile executions, the court noted that most states bar them and those that allow them do so infrequently. Only three states -- Oklahoma, Texas and Virginia -- have executed people who committed crimes as juveniles in the past 10 years.
Legal experts said the ruling could have widespread ramifications for the future of capital punishment, with courts empowered to strike down the practice on evolving notions of decency.
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