I don't think his juvenile status, and the fact that it makes certain of his actions criminal, plays any role in his self defense claims. If it did, what it would amount to is, "If you were eighteen, then what you did would be justifiable self defense, and you would walk free. However, because you are seventeen, what you did is murder, so you will be imprisoned for life."
As I said earlier, I think walking the streets with a loaded rifle ought to be a crime at any age, and I think all these armed yahoos in "militia" groups ought to go home, and I would like to see law enforcement find any excuse they can for locking up the ones who insist on it. However, this is America, and we have the freedom to do some really stupid things that no sane nation would allow, like walk down the street with a loaded rifle. In the middle of a riot, no less.