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Why did the Supreme Court rule that people under age 18 at the time of their crimes cannot be executed?

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Padded gurney with black restraints and IV stands, used for lethal injection, sits in a bare room with white cinder block walls at the New Mexico state penitentiary. — capital punishment Lethal-injection room.

In 2005, the U.S. Supreme Court ruled in Roper v. Simmons that the execution of individuals who were under the age of 18 at the time of their crimes is unconstitutional.1

The Court determined that such executions constitute "cruel and unusual punishment" under the Eighth Amendment, concluding that the death penalty is a disproportionate punishment for juvenile offenders. This decision overturned the Court’s previous 1989 ruling in Stanford v. Kentucky, which had permitted the execution of individuals who were 16 or 17 years old when they committed their crimes.1

The legal reasoning behind these protections is rooted in the principle that punishments must be proportional to the crime and must align with the "evolving standards of decency that mark the progress of a maturing society," a standard previously cited by the Court in Thompson v. Oklahoma (1988) when it prohibited the execution of individuals who were 15 or younger at the time of their offense.23