That's why committing crimes falls into the same category -- the justification behind not punishing juveniles the exact same as adults. The criminal justice system has a number of goals, but when an individual is tried, the primary consideration is supposed to be his culpability. Meaning, how fully formed was the criminal intent, was he capable of understanding the finality and consequences of the action. Which is why retarded and mentally ill people have less culpability. If you can't say an 11-year old has the awareness, maturity, and impulse control to be able to join the military, get married, drink, etc., then it makes no sense that he has the same level of culpability that an adult would have in firing the gun, despite (especially because of) its seriousness.