Final answer:
The Supreme Court decision in Roper v. Simmons prohibits the execution of individuals who were under 18 at the time of their crime.
Step-by-step explanation:
The Supreme Court has held that the Constitution forbids the execution of offenders who were under the age of 18 at the time the crime was committed. This is in line with the decision in Roper v. Simmons, 543 U.S. 551 (2005), which states that a death sentence may not be imposed on juvenile offenders. This limit on the death penalty is an aspect of understanding the constitutional protections against cruel and unusual punishment as stipulated in the Eighth Amendment.