Matthew Simmons was sentenced to death for the murder of a woman when he was 17 years of age. In the 1988 case Thompson v. Oklahoma, the Supreme Court ruled that executing persons for crimes committed at age 15 or younger constitutes cruel and unusual punishment in violation of the Eighth Amendment.
A series of appeals to state and federal courts lasted until 2002, but each appeal was rejected.
Using the reasoning from the Atkins case, the Missouri court decided, 6-to-3, that the U.S. Supreme Court's 1989 decision in Stanford v. Kentucky, which held that executing minors was not unconstitutional, was no longer valid.
In a 5-4 opinion delivered by Justice Anthony Kennedy, the Court ruled that standards of decency have evolved so that executing minors is "cruel and unusual punishment" prohibited by the Eighth Amendment.
The majority cited a consensus against the juvenile death penalty among state legislatures, and its own determination that the death penalty is a disproportionate punishment for minors.