Opponents of capital punishment are applauding this month's ruling by the U.S. Supreme Court outlawing the execution of criminals who were younger than 18 when they committed their crimes. But critics say all juvenile criminals are not the same, and they worry that the Court will expand the ban to cover other classes of offenders.

In a close, five-to-four decision, the Justices of America's highest court ruled that the execution of juveniles convicted of murder violates the Eighth Amendment to the U.S. Constitution, which prohibits cruel and unusual punishment. The ruling overturns laws on the books in 19 the nation's 50 states.

"We think that this decision is long overdue," says Sue Gunawardena-Vaughn -- who directs Amnesty International USA's National Program to Abolish the Death Penalty. She says she supports the court's reasoning that people under age 18, in general, are less morally responsible for their crimes.

"In many areas of the law we treat children differently -- when it comes to buying cigarettes, when it comes to buying alcohol, when it comes to a number of different things," Ms. Gunawardena-Vaughn explains. "So it's inconsistent to say that children are not mature enough to make these decisions until they are 18?but, on the other hand, if they commit a crime under that age, they are eligible for the death penalty."

The five justices who supported the ruling observed that the United States is the only country that still executes offenders under 18. As Justice Anthony Kennedy wrote, "It's proper to acknowledge the overwhelming weight of international opinion against the juvenile death penalty."

In his minority opinion, Justice Antonin Scalia denounced the majority's reliance on international law. Dianne Clements, president of the victims' advocacy group, Justice for All, agrees. She says worldwide rejection of the juvenile death penalty does not justify eliminating it in the United States.

"The United States is the only country that does many things, and that doesn't make us wrong to be a singular nation in what we choose to do," she argues. "So I believe that the United States is right in doing that and the other countries are wrong. I believe that the Court will revisit this at some point in the future."

Another death penalty advocate, New York Law School professor Robert Blecker, criticizes the decision for putting all juvenile criminals into one category. "The irony is this is an indiscriminate move, because it's a categorical exemption rather than a case by case basis," he says. "It's in line with the more general cultural movement, more worldwide movement, that stretches over centuries in the gradual limitation of punishment. But limitation of punishment doesn't mean elimination of punishment."

Legal scholar Todd Gaziano, with the Heritage Foundation, also opposes the new ruling. He told NBC news he is concerned about the consequences of the decision. "There is striking evidence that criminal gangs in America's cities are already employing 15-, 16-, 17-year old assassins to commit the gang's murders in the states that don't execute juveniles," he says.

But executing those young killers doesn't serve the cause of justice, responds Amnesty's Gunawardena-Vaughn. "A young person who is not able to make decisions, if they're going to be manipulated by an adult, they shouldn't be eligible for the death penalty," she argues. "On the other hand, I really don't think there is any evidence, in terms of statistics or more tangible evidence, that, in the states that don't have the juvenile death penalty, crime is in fact higher."

Death penalty advocate Robert Blecker says he is concerned that the ruling wil be expanded in the future to limit or abolish other types of sentences. "I expect three things to happen," he says, "and Amnesty International will lead the way: attack life without parole for the under 18 set, extend the notion of what constitutes a juvenile to?under 21, and attack the execution of the, quote, mentally ill, including in mental illness things like sexual sadism and psychopathy."

Over the past two decades, authorities across America have executed 22 juvenile offenders, with more than half the executions taking place in Texas. This month's Supreme Court ruling tossed out the death sentence of a Missouri man who was 17 when he murdered a woman in 1993.

The decision affects some six dozen juvenile murderers who are currently on death row, and bars states from seeking to execute minors in the future. But it has done nothing to settle the question of who, if anyone, should pay the ultimate price for murder.