Don't Kill Juveniles

What's surprising isn't that the Supreme Court ruled today that it was improper for so many states to apply the death penalty in cases where the offender was under the legal age at the time of the crime.

No, the surprise is that so many states did this in the first place. I believe that most states stipulate that those under the age of 18 are too young to enter into a contract of any kind. If they're too young to do that, why would they be appropriate age to execute - or send to war?