Other than the United States, the only countries believed to have executed juvenile offenders since 1990, according to Amnesty International, are China, the Democratic Republic of Congo, Iran, Nigeria, Pakistan, Saudi Arabia and Yemen. The Supreme Court should remove this country from that infamous company.

The court will hear arguments today over a 2003 Missouri Supreme Court decision that said executing a person who had committed a murder when he was 17 would violate the Eighth Amendment's prohibition against cruel and unusual punishment. The Missouri court was nudging the law in a good direction. The United States Supreme Court ruled in 1989, voting 5 to 4, that it was constitutional to execute people who were 17 at the time of their crimes. But the Missouri court concluded that the 1989 ruling no longer reflected the nation's thinking.

When the Supreme Court considers an Eighth Amendment challenge, it looks to "evolving standards of decency" - and there has been a steady movement nationally away from the juvenile death penalty. In the 15 years since the Supreme Court last considered this question, a significant number of states, including Kansas, Montana, Wyoming, South Dakota and New York have prohibited the execution of juvenile offenders. In 30 states and the District of Columbia, there now is either no death penalty, or the death penalty applies only to people who were 18 or older at the time of their crimes. Even in those states where juvenile offenders can be executed, it is extremely rare. Only three states have done so in the past decade.

When it considers this case, the court should give weight to the growing scientific literature that says young people's brains are still developing in important ways before the age of 18, and to the nearly unanimous international opinion on this issue.

On the same day in 1989 that the court upheld the death penalty for juvenile offenders, it ruled that the mentally retarded could be executed. But in 2002, the court reversed itself, concluding that national standards of decency had evolved away from permitting the execution of the mentally retarded. The court should reach the same conclusion now for juvenile offenders.

The day will no doubt come when five justices will reach the conclusion that the death penalty is unconstitutional in all cases. But as long as the court is approving capital punishment for some defendants and not others, it should hold that 17- and 18-year-old offenders fall on the wrong side of the line.