The point isn't whether or not it's right to execute brutal murderers if they were under 18. The point is whether this is a constitutional issue, under the phrase "cruel and unusual punishment." I think not.
If it's wrong to execute murderers under 18, then this should be a matter for the voters and the legislatures.
No, it is for that reason that the Eighth Amendment was written and the courts apply it.
Given the rhetoric, all sorts of cruel and unusual punishments would be instituited through popular vote or representative vote. Doesn't make it right, though - as the popular canard goes, "Democracy is two wolves and a sheep voting on what's for dinner."
The law makes clear distinctions between adults and children, and making children ineligible for capital punishment is not totally out of left field.
If one believes that children ought to be liable for capital punishment, then one ought to also beleive that there should be no distinctions based on age, or mental maturity, in the criminal law. Say goodbye to statutory rape laws, then, as just one example, as they are predicated entirely on punishing those who take advantage of mental immaturity...