EDIT: People seem to be misreading me. I said that the justice system drawing an arbitrary line at 18 years of age seems odd, considering some children are mature and some aren't. Some children know what they're doing. Some don't. Why is 18 optimal? Is it just the least bad solution to a problem that has no good solutions?
You must get that this boy is not even a teenager yet. Destroying his life by throwing him into a prison will only result in him learning and befriending other troubled children. This will not do anyone any good.
The solution I would suggest is a minor punishment, say a week or so in prison - just to scare the living hell out of him. Meanwhile having a professional evaluate his situation and suggest a reasonable recovery plan (therapy, forced change of school in order to find new friends, computer banishment, or whatever).
Don't twist the statements here. I guess a discussion about arbitrary cutoffs for 'Yeah, a month ago he didn't know what he was doing, but now he's mature' would be fine.
To me, this is what it is. Determining maturity and adulthood in an institutionalized fashion with a subjective measurement would be extremely difficult. Drawing a line in the sand at 18 wildly simplifies the problem of maturity, around the time when most children's brains have mostly finished developing. As far as objective measures for when you should be considered an adult, "your brain is done developing, you now have an adult brain" seems fairly reasonable.