Which case confirmed that executing individuals under 18 is unconstitutional?

The case that confirmed executing individuals under the age of 18 is unconstitutional is Roper v. Simmons. This landmark Supreme Court decision, issued in 2005, highlighted the evolving standards of decency that inform the application of the Eighth Amendment's prohibition against cruel and unusual punishment. The Court concluded that juveniles are categorically less culpable than adults due to their lack of maturity, underdeveloped sense of responsibility, and vulnerability to external influences. Thus, executing minors was deemed incompatible with contemporary standards of decency.

This ruling is foundational in juvenile law as it recognizes that children and adolescents are fundamentally different from adults in terms of their development and moral reasoning. The decision underscores the importance of rehabilitation over punishment for juvenile offenders, shaping how the justice system treats young individuals.