Why does youth (age) matter in determining the appropriateness of a lifetime of incarceration without the possibility of parole.

Defendant was convicted in the Alabama Circuit Court of capital murder committed when he was 14 years old. Defendant appealed his conviction and the resulting sentence of life in prison without possibility of parole. In Miller v. Alabama, the U.S. Supreme Court held that held that mandatory life imprisonment without parole for those under the age of 18 at the time of their crimes violates the Eighth Amendment's prohibition on cruel and unusual punishments.