ROPER, Superintendent, Potosi Correctional Center v. SIMMONS
543 U.S. 551, 125 S.Ct. 1183 (2005)
At age 17 Simmons planned and committed a capital murder. After he had turned 18, he was sentenced to death. His appeals were rejected.
Then US Supreme Court decided Atkins v. Virginia, Simmons filed a new petition arguing that Atkins' reasoning applied to juveniles as well as the Retarded. The Missouri Supreme Court agreed and set aside Simmons' death sentence in favor of life imprisonment without eligibility for release. The US SC reviewed and held that even though it had rejected in 1989,(Stanford v. Kentucky) the proposition that the Constitution bars capital punishment for juvenile offenders younger than 18, standards of decency had evolved and a national consensus has developed against the execution of those offenders since.
The Eighth and Fourteenth Amendments forbid imposition of the death penalty on offenders who were under the age of 18 when their crimes were committed.