You can't say that a fifteen year old doesn't understand that if they shoot someone in the back, there is a strong possibility they may die (or at least suffer extraordinary injury). 15 year olds are in high school. They are studying geometry and chemistry. They are more than capable of understanding that guns are not toys and that shooting someone for no reason means they are likely to be put away for life.
It's one thing if a kid robs somebody. Just because you fell into the wrong crowd and stole something doesn't mean you are incapable of becoming a responsible member of society without having to go through the prison experience. I'm sorry, if you shoot somebody in the back as a 15 year old, you need to be put in a mental institution and/or prison, because there is something seriously wrong with you.