Death penalty ruling

The SCOTUS decreed the death penalty unconstitutional for those who commit crimes when under 18. I'm looking for the passage in the Constitution that indicates 'cruel and unusual' is somehow different based on age, but can't find it.

Good for us, though, Justice Kennedy clearly understands the role of the international community on our Constitution:

"It is proper that we acknowledge the overwhelming weight of international opinion against the juvenile death penalty, resting in large part on the understanding that the instability and emotional imbalance of young people may often be a factor in the crime."

Huh? I can't seem to find the part of the Constitution that talks about doing what the international community likes. Of course, getting a copy of the Constitution is quite challenging, and the language used isn't always as clear as it could be, so maybe somewhere in there (probably near the 'right to an abortion' clause, or the part where it says it doesn't apply to people who the government says it doesn't) it says something about this, and I'm just too stupid to see it.

Full disclosure: I am against the use of the death penalty under any circumstances, as it is the ultimate tyranny. However, I can see a Constitutional argument for both sides. I just wish the SCOTUS would base it's rulings on that document.