The Missouri Supreme Court today vacated the death sentence of Christopher Simmons, holding that the imposition of the death penalty on those who were juveniles at the time of the crime violates the 8th Amendment ban against cruel and unusual punishment. Simmons was 17 at the time of the murder for which he was sentenced to death. His sentence will now become one of life without parole or release, except by an act of the Governor.

From the Court syllabus:

(1) In 1989, the United States Supreme Court declined to bar the execution of offenders who were 16 or 17 years old at the time of their crimes, holding that there was not then a national consensus against such executions. Stanford v. Kentucky, 492 U.S. 361 (1989). This Court is not bound by Stanford, however, because the determination of what is cruel and unusual punishment under the Eighth Amendment must be interpreted in a flexible and dynamic manner. This determination must be considered by current standards, recognizing that society's standards of decency are fluid and evolving.

(2) In the 14 years since Stanford was decided, a national consensus has developed against the execution of juvenile offenders. No state has lowered the age for execution from 18 to 17 or 16, five more states have banned the practice of executing juvenile offenders through legislative action and a sixth state has banned such a practice through a judicial decision. Only six states have executed a juvenile offender in the past 14 years. Opposition to the juvenile death penalty by professional, social and religious organizations, both nationally and internationally, has grown since Stanford. Similar to the reasons set out in Atkins in regard to offenders who are mentally retarded, neither retribution nor deterrence provides an effective rationale for imposing the juvenile death penalty, and the risk of wrongful execution of juveniles is enhanced.

(3) This Court concludes that the United States Supreme Court would hold that the execution of persons for crimes committed when they were under 18 years of age violates the evolving standards of decency and is prohibited by the Eighth Amendment to the United States constitution. This decision applies retroactively to persons whose cases are on collateral review.

You can read the syallbus and full opinion here.