Opponents of capital punishment won a major victory at the U.S. Supreme Court Tuesday.  By a vote of five to four, the high court ruled that it is now unconstitutional to execute criminals who were younger than 18 when they committed their crimes.

The majority opinion striking down juvenile death sentences was written by Justice Anthony Kennedy.  He noted what he called the "overwhelming weight of international opinion" that has moved against the juvenile death penalty in recent years.

Justice Kennedy also wrote that American society views juveniles as, in his words, "categorically less culpable than the average criminal" because of their lack of maturity and emotional stability.

Reaction from death penalty opponents was swift and positive.

Marsha Levick is with a group called the Juvenile Law Center in New York.

"We cannot impose capital punishment on offenders who do not demonstrate the same degree of adult blameworthiness, adult culpability and mature judgment as adult offenders do," said Ms. Levick.

The narrow five to four decision affects 19 states that had allowed offenders under the age of 18 to be put to death. The ruling abolishes death sentences for more than 70 murderers now on death row who committed their crimes when they were 16 or 17.  The Supreme Court outlawed executions for those 15 and younger in 1988.

The United States had been among a dwindling handful of countries in recent years that still permitted juvenile executions.  Other nations that have been moving to end the practice include Iran, Pakistan, China and Saudi Arabia.

Richard Dieter is the executive director of the Death Penalty Information Center, a group based in Washington that opposes capital punishment.

"What Justice Kennedy is saying is that if we look to serving in the military, voting, drinking, age 18 is a bright line,? he noted.  ?Surely, we should apply the same standard to those who can be executed."

Justice Sandra Day O'Connor wrote a dissenting opinion for the four-member minority.  She argued that even though those under 18 may be less culpable for their crimes than adults, they should still be eligible for the death penalty depending on the severity of the offense.

The decision was not welcome news for family members who have lost loved ones to teenage murderers.  Bill Green and Adolph Pena both lost children to killers who committed murder at the age of 17.

GREEN:  "The person who killed our son had already committed over 14 felonies by the time he was 17 years old."

PENA:   "I can tell you one thing.  These 17-year-olds knew what they were doing when they murdered my daughter."

The Death Penalty Information Center says 22 criminals have been executed for crimes committed when they were 16 or 17 since the Supreme Court reinstated the death penalty in 1976.  

The latest ruling continues a trend on the Supreme Court in recent years to restrict the use of the death penalty.  The high court banned the execution of mentally retarded criminals in 2002, calling it a violation of the U.S. Constitution's ban on cruel and unusual punishment.