John Penry was subsequently convicted of rape but sentenced to only 5 years in prison. He was paroled after just two years. After his conviction, Mr. Penry was sentenced to die by lethal injection. This set the stage for one of the Supreme Court's most important capital cases of the 20th century. With that 1989 decision, the Court ruled that it is not unconstitutional to execute a mentally retarded inmate.
Mental age is directly relevant to moral culpability.
Chronological age is merely a proxy. The Court's decision defies both common sense and common decency. Even if one rejects the notion that there should be an absolute rule prohibiting the execution of the profoundly mentally retarded, there is another argument for sparing Mr. Penry's life. Currently, only a small percentage of those who kill end up being executed. I would argue that if we executed 99 killers for every one that we spared, Mr. Penry would probably be the best candidate among the 100 for a life sentence.