The United States Supreme Court issued its decision in Roper v. Simmons (March 1, 2005). The Court held, in a 5-4 ruling, that the Eighth and Fourteenth Amendments forbid imposition of the death penalty on offenders who were under the age of 18 when their crimes were committed.
Justice Kennedy writes (excerpt):
It is difficult even for expert psychologists to differentiate between the juvenile offender whose crime reflects unfortunate yet transient immaturity, and the rare juvenile offender whose crime reflects irreparable corruption. See Steinberg & Scott 1014-1016. As we understand it, this difficulty underlies the rule forbidding psychiatrists from diagnosing any patient under 18 as having antisocial personality disorder, a disorder also referred to as psychopathy or sociopathy, and which is characterized by callousness, cynicism, and contempt for the feelings, rights, and suffering of others. American Psychiatric Association, Diagnostic and Statistical Manual of Mental Disorders 701-706 (4th ed. text rev. 2000); see also Steinberg & Scott 1015. If trained psychiatrists with the advantage of clinical testing and observation refrain, despite diagnostic expertise, from assessing any juvenile under 18 as having antisocial personality disorder, we conclude that States should refrain from asking jurors to issue a far graver condemnation–that a juvenile offender merits the death penalty. When a juvenile offender commits a heinous crime, the State can exact forfeiture of some of the most basic liberties, but the State cannot extinguish his life and his potential to attain a mature understanding of his own humanity.