Which U.S. Supreme Court case held that executing juveniles is unconstitutional?

Roper v. Simmons held that executing people for crimes committed before they turned 18 violates the Eighth and Fourteenth Amendments.

The 2005 decision concerned Christopher Simmons, who was 17 when he planned and committed a murder in Missouri. He was convicted and sentenced to death. While his appeals were pending, the Supreme Court decided Atkins v. Virginia, which barred execution of people with intellectual disability. Simmons challenged his sentence, eventually bringing the juvenile-execution question back to the Court.

By a 5–4 vote, the Court concluded that juveniles have diminished culpability because of immaturity, vulnerability to outside influence, and an incompletely formed character. Justice Anthony Kennedy's majority opinion also pointed to an emerging national consensus against juvenile capital punishment and the practice's rarity internationally. The ruling made age 18 the constitutional cutoff for crimes committed by juveniles.

Thompson v. Oklahoma had already barred execution for crimes committed under age 16, while Stanford v. Kentucky had allowed execution for some 16- and 17-year-olds. Roper superseded that distinction and established the broader rule.