I understand that, and 12 is not as developed as 18, which is an adult. They cannot vote, drive, join the army, buy cigarettes, decide who they can have sex with, if they have to go to school or not, or anything else that only adults can do. If you think 12 years old understand enough about the world to vote, drink, and fuck, then let's do that before we start killing them for retribution or locking them up for life.
Not only that, they are sure as hell not competent enough to understand the court process, which is another reason for the juvenile court system.