The court decides such treatment is "cruel and unusual."
In a 5-4 decision, the U.S. Supreme Court deemed mandatory life sentences for juveniles a violation of the Constitutional ban on cruel and unusual punishment as written into the 8th Amendment. Justice Elena Kagan writes:
By making youth (and all that accompanies it) irrelevant to imposition of that harshest prison sentence, such a scheme poses too great a risk of disproportionate punishment.\n
The court did not, however, categorically bar all life sentences without parole for juveniles, it eliminated the ability of states to mandate the most severe penalty for juvenile murder. And states can still load juveniles down with very, very long sentences that aren't technically life sentences.