The Supreme Court established in Roper v. Simmons that executing people who were under 18 at the time of their crimes violates the Eighth Amendment. In Atkins v. Virginia, it banned executing people with intellectual disabilities. In Kennedy v. Louisiana, it prohibited execution for non-homicide crimes.
These protections exist on paper but enforcement varies widely. Courts prioritize procedural rules that can prevent defendants from proving intellectual disability or other mitigating factors. States continue to schedule executions in cases where experts agree the defendant meets the constitutional criteria for exemption. The gap between constitutional standards and actual practice means your protection against cruel punishment depends heavily on which state you're in, which judge hears your case, and whether your lawyer raises the right arguments at the right time.