Find the Exception: Saudi Arabia, Pakistan, US, China
Today, it is now the US, as SCOTUS has ruled that excecuting people under 18 when they committed a crime is unconstitutional.
Excellent news from an international relations point, as we can now join several treaties and global standards about the rights of a child. We are worldwide leaders in several standards about rights, but while still executing children we are somewhat lacking in our moral authority to convince other nations to sign the treaty.
Also good: we no longer kill children.
That being said, it is disgusting that we needed the Supreme Court to tell us this, and bad for a number of reasons. Reading the opinions, it is clear that subjective standards are responsible here and certainly no objective or strict judgement from the constitution. Justice Scalia pointed out "The court says in so many words that what our people's laws say about the issue does not, in the last analysis, matter: 'In the end our own judgment will be brought to bear on the question of the acceptability of the death penalty,"' When (in my opinion, not if) this reasoning eliminates the death penalty altogether (again), we can guarantee frustrated culture wars flaming up all over again.
Of course, we have an amendment that says "no cruel or unusual punishment" which our political system has decided that it is the responsibility of courts to enforce, not our democratic represenatives. As such, they do have to employ subjective decisions about what is cruel and unusual. Which is sad to leave up to unelected unaccountable justices.
No I don't think this is a matter of the Constitution doing good, btw. It's pretty clearly federalism that is responsible for executing juveniles in the first place. Only 19 relatively depopulated (edit: no, avg pop) states execute juveniles. Not even enough to hold a filibuster. The federal government does not execute that group, and I'm pretty sure if the federal government wrote all the rules on the death penalty instead of states, it would not be an issue.
PS: Is a 17 year old gang member a "child"? Excellent points can be made that in previous societies age of maturity was as early as 12. And current standards are only in place because of a) a desire to increase the high skill labor supply through more education and b) a desire to decrease the low skill labor market. Tough question, but as a society we clearly do set a standard at 18 for a number of reasons, and I sure as hell don't want us killing or sending to war anyone who couldn't vote no against the guys who decided that.