At age 17, respondent Simmons planned and committed a capital murder. After he had turned 18, he was sentenced to death. His direct appeal and subsequent petitions for state and federal postconviction relief were rejected. This Court then held, in Atkins v. Virginia, 536 U. S. 304, that the Eighth Amendment, applicable to the States through the Fourteenth Amendment, prohibits the execution of a mentally retarded person. Simmons filed a new petition for state postconviction relief, arguing that Atkins' reasoning established that the Constitution prohibits the execution of a juvenile who was under 18 when he committed his crime. The Missouri Supreme Court agreed and set aside Simmons' death sentence in favor of life imprisonment without eligibility for release. It held that, although Stanford v. Kentucky, 492 U. S. 361, rejected the proposition that the Constitution bars capital punishment for juvenile offenders younger than 18, a national consensus has developed against the execution of those offenders since Stanford.

Held: 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. Pp. 6-25.

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