We have a Constitution in this country that regulates government power. Government does not have the power to torture us, for instance.
In that spirit, we must think of jailing someone as a very weighty and significiant thing to do. The jailing of a man is a very serious thing to do. You act as if it is not, and that somehow the state is giving him the "gift" of room and board. The state has denied the prisoner the means of making a living for himself, and thus is obligated to make sure that he has the basic things he needs to survive.
On juveniles, the reference to changing global standards on juvenile execution and the interpretation of the 8th amendment in a recent supreme court opinion is, I believe, justified. The 8th amendment is an amendment that even an Orginalist should believe changes in meaning over time. The phrase "cruel and unusual" was not slang for a given set of practices, it references current (whenever current is) thinking on what "cruel and unusual punishment" means. Modern thinking must be applied to this definition. A juvenile may understand his actions, but, under the Constitution, I believe he cannot be executed.