In an opinion authored by Justice Kennedy, the Court wrote:
The differences between juvenile and adult offenders are too marked and well understood to risk allowing a youthful person to receive the death penalty despite insufficient culpability. An unacceptable likelihood exists that the brutality or cold-blooded nature of any particular crime would overpower mitigating arguments based on youth as a matter of course, even where the juvenile offender's objective immaturity, vulnerability, and lack of true depravity should require a sentence less severe than death. In some cases a defendant's youth may even be counted against him. In this very case, as we noted above, the prosecutor argued Simmons' youth was aggravating rather than mitigating. While this sort of overreaching could be corrected by a particular rule to ensure that the mitigating force of youth is not overlooked, that would not address our larger concerns.
The Court also took note of international trends away from the use of the death penalty on minors.
Court's full opinion
Justice O'Connor's dissent
Justice Scalia's dissent
joined by Chief Justice Rehnquist and Justice Thomas
Supreme Court Ends Execution of Juvenile Murderers
Tue Mar 1, 2005
WASHINGTON (Reuters) - A closely divided U.S. Supreme Court on Tuesday abolished the death penalty for those who were under the age of 18 when they committed murder, a major victory for opponents of capital punishment.
By a 5-4 vote, the high court declared unconstitutional the juvenile death penalty, a decision that could affect about 70 death row inmates who face execution for crimes done when they were 16 or 17 years old.
The decision amounted to a significant change from the Supreme Court ruling 16 years ago when it held the execution of such juvenile offenders did not violate the constitutional ban on cruel and unusual punishment.
Opponents of capital punishment had argued that world opinion and a national consensus has now formed against the juvenile death penalty and said it should be struck down as unconstitutional, like the Supreme Court did in 2002 in barring executions of mentally retarded criminals.
In the court's majority opinion, Justice Anthony Kennedy agreed and declared the U.S. Constitution forbids the imposition of the death penalty against offenders who were under the age of 18 when their crimes were committed.
"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," he wrote in the 25-page opinion.
Chief Justice William Rehnquist and Justices Sandra Day O'Connor, Antonin Scalia and Clarence Thomas dissented from the ruling.