Thompson v. Oklahoma
At the age of 15 years Thompson was tried as an adult, convicted of first degree murder, and sentenced to death. On appeal, the Court of Criminal Appeals of Oklahoma affirmed. The Supreme Court granted Thompson certiorari.
Would the execution of a 15 year old violate the Eighth Amendment's prohibition against "cruel and unusual punishments"?
Yes. After noting that the Eighth Amendment's prohibition against "cruel and unusual punishments" applied to the states through the Fourteenth Amendment, the Court held that the execution of a person under the age of 16 was unconstitutional. In noting the uniform ban among all relevant state statutes against the execution of one under the age of 16, the Court explained that such an act would violate the "evolving standards of decency that mark the progress of a maturing society." The case was reversed and remanded.

