The U.S. Supreme Court (search) on Tuesday ended the option of the death penalty for 16- and 17-year-olds, stating that persons of that age cannot "be classified among the worst offenders."

The 5-4 decision in Roper v. Simmons (search) affects 19 states that still subject youths to capital punishment, along with 72 death-row inmates who committed their crimes as teenagers.

In a somewhat surprising development, Justice Sandra Day O'Connor, previously seen as the "swing" voter in the case, joined Chief Justice William H. Rehnquist and Justices Antonin Scalia and Clarence Thomas in a dissent, while Justice Anthony Kennedy sided with the majority.

"When a juvenile offender commits a heinous crime, the state can exact forfeiture of some of the most basic liberties, but the state cannot extinguish his life and his potential to attain a mature understanding of his own humanity," Kennedy wrote in the majority ruling.

Kennedy, who in 1989 voted in favor of upholding the death penalty for 16- and 17-year-olds, cited the court's requirement that it consider "evolving standards of decency that mark the progress of a maturing society" in the reversal of its previous ruling on capital punishment for juveniles.

Moreover, Kennedy implied that Tuesday's decision may have been overdue.

"The impropriety of executing juveniles between 16 and 18 years of age gained wide recognition earlier than the impropriety of executing the mentally retarded," he concluded after comparing the histories of capital punishment for retarded and underage offenders.

The court struck down the death penalty for the mentally retarded in 2002.

Justices David H. Souter, John Paul Stevens, Stephen G. Breyer and Ruth Bader Ginsburg also voted with the majority.

The five affirming justices based their opinion on the Eighth and Fourteenth Amendments — specifically, on the prohibition of cruel and unusual punishment.

The offender in question in the Supreme Court case, Christopher Simmons (search), robbed and killed a 46-year-old Missouri woman in 1993, when he was 17. He is now 29.

While a 1998 high court decision prevented his 15-year-old accomplice from being tried for capital murder, Simmons was convicted and sent to death row.

The court's decision marks the third major restriction on the death penalty in 17 years, and may be seen by advocates as moving the United States closer to a full abolition of capital punishment.

Juvenile justice advocates said they were "delighted" with the court's decision.

"This is a huge win for us," said Bryan Stevenson, executive director of the Equal Justice Initiative of Alabama. "The political culture is so committed to appearing rough you can never be too confident in what the law requires."

Alabama has 14 inmates on death row who committed their crimes as juveniles, the second-highest number next to Texas, which has 28 juvenile offenders awaiting death and has executed 13 since 1976.

Texas state Rep. Lon Burnam said a bill he sponsored raising the age of eligibility to 18 is set for review next week. While admittedly a symbolic exercise after Tuesday's decision, the Democrat said his colleagues want the opportunity to bring Texas law in line with the court.

"I wouldn't say the tide is turning, but certainly people are embarrassed and frustrated to realize that we are so out of step with the rest of the world," Burnam said. "We look pretty barbaric compared to the rest of the world."

Burnam said he supports a nationwide moratorium on capital punishment in light of several high-profile cases of death-row inmates who were exonerated by DNA and other kinds of evidence.

Marsha Levick, legal director of the Juvenile Law Center in Philadelphia, said she saw broader implications for how juvenile justice is practiced in America.

"The court has recognized that juveniles are categorically distinct from mature offenders," Levick said. "It's up to us as judges, advocates, to explore those implications with regard to other forms of sentencing, the way they are treated in society."

But, she emphasized, juvenile offenders "can and should be punished in developmentally appropriate ways. [Teen Beltway sniper] Lee Malvo (search) will not get out of prison as a result of this decision."

Paul Ebert, Prince William County (Va.) Commonwealth's attorney, had said he wanted to seek the death penalty for Malvo but was waiting on the Roper decision.

"Based on this decision, we don't intend to proceed against him," Ebert said. "Should a future court reverse this decision we very well may go forward."

A plea deal won Malvo a life imprisonment sentence for the 2002 Beltway sniper shootings.

Virginia, Texas and four other states filed an amicus brief for Roper in which they argued that states must have the option to apply the most severe punishment to underage offenders, even if their numbers were few. Justice O'Connor was apparently swayed by their argument.

"At least some 17-year-old murderers are sufficiently mature to deserve the death penalty in an appropriate case," O'Connor wrote in her dissent.

Robert Blecker, a professor of criminal law at New York Law School, worried that death-penalty opponents would use the Roper decision to further chip away at fair punishments for the worst offenders.

Blecker predicted that opponents would next go after life imprisonment without parole for juveniles, would lobby to raise the age of eligibility to 20 or 21 and would argue against the death penalty for the mentally ill.

"Think of the irony. They previously argued for life without parole on the same grounds — juvenile offenders' character isn't fully formed, etc.," he said.

Blecker also cited the case of Connecticut serial killer Michael Ross, who is at the center of a legal tug-of-war over whether he can be executed.

"He murdered eight women and raped seven of them first ... then he was diagnosed with sexual sadism. Is that [his crime] sick? No, that's evil," Blecker argued.

Justice Scalia insisted that some acts were simply too horrifying to be mitigated, no matter the perpetrator's age or mental capability.

"Exceptional cases" must be approached in an "exceptional way," Scalia wrote in his dissent. "Some murders are not just the acts of happy-go-lucky teenagers, but heinous crimes deserving of death."