Kennedy's opinion rested in large part on the fact that 30 states, including the 12 states that have no capital punishment, forbid the death penalty for offenders younger than 18. That number represented an increase of five since the court upheld the juvenile death penalty in 1989.
The court weighs death penalty laws according to what a 1958 ruling called the "evolving standards of decency that mark the progress of a maturing society," and looks to state legislation and jury verdicts to decide whether a "national consensus" has developed against a previously accepted practice.
Christopher Simmons, age 17 when he kidnapped and killed a woman, was spared along with 72 others.
In 2002, the court voted 6 to 3 to strike down the death penalty for the moderately mentally retarded, which it had upheld 5 to 4 in 1989. In the 2002 case, Atkins v. Virginia, the court noted that the number of death penalty states banning that practice had grown from two in 1989 to 13 in 2002, while none had gone the other way.
The recent shift of states against the juvenile death penalty, though less dramatic than the evidence the court found sufficient in the mental-retardation case, was enough to carry the day, Kennedy concluded.
For the Supreme Court itself, perhaps the most significant effect of yesterday's decision is to reaffirm the role of international law in constitutional interpretation.
The European Union, human right lawyers from the United Kingdom and a group of Nobel Peace laureates had urged the court in friend-of-the-court briefs to strike down the juvenile death penalty.
In saying that this strong expression of international sentiment "provide[s] respected and significant confirmation for our own conclusions," Kennedy lengthened the recent string of decisions in which the court has incorporated foreign views -- and decisively rejected the arguments of those on the court, led by Scalia, who say it should consider U.S. law exclusively.
There were actually six votes in Kennedy's favor on that point yesterday, because in her dissenting opinion O'Connor agreed with Kennedy that international trends affect the meaning of "cruel and unusual punishment" in modern times.
O'Connor's opinion suggested she came fairly close to joining the majority entirely. If she were a legislator, O'Connor wrote, "I, too, would be inclined to support legislation setting a minimum age of 18 in this context."
But, O'Connor wrote, too few states had recently enacted such laws to convince her that the country generally had "set its face" against the juvenile death penalty.
Scalia, in a separate dissent joined by Rehnquist and Thomas, took the majority to task for "proclaim[ing] itself sole arbiter of our Nation's moral standards -- and in the course of discharging that awesome responsibility purport[ing] to take guidance from the views of foreign courts and legislatures."
Noting that most countries have more restrictive abortion laws than the United States, Scalia accused the court of "invok[ing] alien law when it agrees with one's own thinking, and ignor[ing] it otherwise." He read his opinion from the bench, a sign of strong disapproval for the court's decision.
Scalia also pointed out that the 18 death-penalty states that limit capital punishment to offenders 18 and older amount to 47 percent of the 38 death-penalty states.
"Words have no meaning if the views of less than 50 percent of death penalty States can constitute a national consensus," he wrote.
For Kennedy, yesterday's opinion appeared to represent a distance traveled since the 1989 case, in which he voted with Scalia to uphold the juvenile death penalty.
As recently as April 2003, the court -- with Kennedy's support -- granted Oklahoma's request to reinstate the death sentence of a 17-year-old offender after a federal appeals court had blocked it.
In 2002, the court refused to hear two appeals from younger-than-18 offenders asking it to reconsider their cases in light of Atkins. Again, Kennedy was in the majority.
Even at the Oct. 12 oral argument in the case decided yesterday, Kennedy said he was "very concerned" that gangs might use juveniles as "hit men" if there were no death penalty.
But yesterday's packet of opinions contained a brief writing by Stevens, co-signed by Ginsburg, that patted Kennedy on the back for coming around to their point of view.
If the "great lawyers" of the early republic were on the court today, Stevens wrote, "I would expect them to join Justice Kennedy's opinion for the court."
The case is Roper v. Simmons, No. 03-633.