Roper v. Simmons, Supreme Court of the United States, March 1, 2005 [striking down by 5-4 the death penalty for juvenile killers as unconstitutional]. Excerpt [from the majority opinion by Justice Kennedy}:

The differences between juvenile and adult offenders are too marked and well understood to risk allowing a youthful person to receive the death penalty despite insufficient culpability. An unacceptable likelihood exists that the brutality or cold-blooded nature of any particular crime would overpower mitigating arguments based on youth as a matter of course, even where the juvenile offender's objective immaturity, vulnerability, and lack of true depravity should require a sentence less severe than death. In some cases a defendant's youth may even be counted against him. In this very case, as we noted above, the prosecutor argued Simmons' youth was aggravating rather than mitigating. While this sort of overreaching could be corrected by a particular rule to ensure that the mitigating force of youth is not overlooked, that would not address our larger concerns.

Read the court's full opinion [PDF]. Reported in JURIST's Paper Chase here.