Roper v. Simmons, 543 U.S. 551 (2005) was a decision in which the Supreme Court of the United States held that it is unconstitutional to impose capital punishment for crimes committed while under the age of 18. The 5-4 decision overruled the Court's prior ruling upholding such sentences on offenders above or at the age of 16, in Stanford v. Kentucky, 492 U.S. 361 (1989), overturning statutes in 25 states that had the penalty set lower.
Read more about Roper V. Simmons: The Case, The Ruling, The Dissents