While I thoroughly agree with Scalia on this, the math of 18 of 38 opposing the youth death penalty IS a consensus.
They forget that the other 12 States don't approve any death penalty at all. The consensus, then is 30 of 50 disapprove of the youth death penalty.
The question, though, is whether there is anything in the Constitution that says the death penalty is cruel and unusual punishment.
Since the Constitution affirms that one may be deprived of life via due process of law, then it follows that the death penalty is not necessarily cruel and unususal.
I don't see how you can say that 30 of 50 is a consensus. Of course, the WSJ points out that the Constitution doesn't automatically amend itself everytime there is a consensus on something anyway. The abortion decision is the killer argument on that. As they point out, every state had some law regulating abortion when the Court issued Roe.