While I am opposed to the death penalty for all, from which it follows that I oppose the death penalty for juvenile offenders, your post points out just how unclear the morality of that particular question is for those who do not rule out the death penalty entirely (as today's court declined to do, though it's not clear why, since it is not the Constitution that was guiding them). Thus, one is left with the "consensus" that even Kennedy is not too confident in claiming, bolstered (by his own admission) by Kennedy's morally superior reasoning. So that's your answer, Phocion--you have your view and you are wrong and in our Constitutional system Kennedy and the other four in the majority simply get to dictate that "consensus" for all of us.
As Scalia also notes, the very notion that juvenile murderers are always and everywhere incapable of the kind of moral reasoning requisite to make them liable for the death penalty is rebuked by the same justices in their abortion jurisprudence. So pregnant teenage girls can think through the difficult choice of abortion but the murderer in today's case was just an innocent babe. Perhaps they should send along their moral psychology to Larry Summers for analysis--at least it comes out better for the fairer sex. Sadly, it is equally ill-conceived.
So while I can be happy that this event might advance the society's moral reasoning with respect to the death penalty, I lament that it has to come at the price of more judicial usurpation and mangling of history, precedent, and the rule of law.
Finally, the title of my post is not entirely without purpose: the Rope-a-dope strategy employed by Ali against Foreman in 1974's "Rumble in the Jungle" is what the anti-death penalty majority is up to, I think. They will be content to write opinions that take the body blows of Scalia's best stuff in piecemeal decisions like this one and Atkins assuming that the gradual approach will eventually exhaust any political opposition. I can't wait until the Sages tell us which moral consensus we've arrived at next!