Washington, March 1 (Reuters): The US Supreme Court today abolished the death penalty for juveniles, a major victory for opponents of capital punishment in the last country in the world that gave official sanction to the execution of people who commit crimes as minors.

By a 5-4 vote, the court declared unconstitutional the death penalty for those under the age of 18 when they committed their crimes, a decision that could affect more than 70 death row inmates who face execution for murders done when they were 16 or 17 years old.

The decision amounted to a significant change from the Supreme Court ruling 16 years ago when it held the execution of juvenile offenders did not violate the constitutional ban on cruel and unusual punishment. The Amnesty International human rights organisation, which has campaigned for years against the execution of child offenders, hailed the ruling.

Opponents of capital punishment had argued that world opinion and a national consensus has now formed against the juvenile death penalty and said it should be struck down as unconstitutional, like the Supreme Court did in 2002 in barring executions of mentally retarded criminals.

In the court?s majority opinion, Justice Anthony Kennedy agreed and declared the US constitution forbids the imposition of the death penalty against offenders who were under the age of 18 when their crimes were committed.

?It is proper that we acknowledge the overwhelming weight of international opinion against the juvenile death penalty, resting in large part on the understanding that the instability and emotional imbalance of young people may often be a factor in the crime,? he wrote in the 25-page opinion.

Kennedy cited evidence of a national consensus against the death penalty for juveniles.

?Neither retribution not deterrence provides adequate justification for imposing the death penalty on juvenile offenders,? he said.