Yesterday in Roper v. Simmons , the Supreme Court decided that the people of 18 states cannot pass laws to execute minors. Yes, that's the proper way to frame this issue. As Justice Scalia pointed out in his dissent, the people of the states should be allowed to make this decision, not five unelected judges.
This decision is just another in a long line of rulings in which the Supreme Court usurped the power of the people by finding rights that do not exist, or finding moral reasons to overturn laws.
Justice Kennady argued that the consensus of US opinion is against executing minors. I'm not sure how 32 states is a big enough consensus to overturn laws. Justice Scalia did a great job explaining how there really isn't a consensus in his dissent.
But why should a consensus be important anyway? Do we not believe that states have a right to make its own laws providing they don't contradict federal law or the Constitution? How can we determine whether states are violating the Consituion by looking to see if other states approve? Does that not undermine federalism?
The Court believes we must determine what is cruel and unusual by the evolving standards of society. This very standard gives the Court almost unlimited discretion in making moral determinations (even based on the consensus of the "international community") and imputing them into the Constitution.
Liberals really like to analyze this case as if it was a bill that was passed. See here (focusing on whether we should listen to world opinion), here (challenging Justice Scalia's point that it's inconsistent to state that minors are able to make complex moral decisions about abortion but not fairly straightforward decisions such as not murdering) and here (glad that the Court decided to do the "civilized thing.")
None of these issues are remotely relevant in crafting a legal decision based on the Constitution. The Court cannot look to see if the laws are "civilized" or in accord with the world. In can only look to see if the Constitution prohibits it. How can the Constitution prohibit something it explicitly allowed when the amendment was passed?