If you shoot and injure (or kill) a 15 year old running across your lawn to get to his house, even if he stopped to look at your prized rose collection - or your daughter sunbathing - you will be charged and most likely convicted.
If you shoot and injure (or kill) a 15 year old who has walked in your back door in the middle of the night, but is otherwise unarmed and not a direct threat, you will be charged. Whether you go to jail or not will depend on a lot of factors, but you will probably also be a defendant in a civil suit which, if case law is any guide, you *will* lose.
If you shoot and injure (or kill) a 15 year old who is running at you with a knife and appears intent on causing you bodily harm, you will be charged. But you will also be allowed to argue that you were acting in self-defense. If the jury believes you, you will go free.
It's how the legal system in the US works. If you didn't learn this in high school, you simply didn't listen to what was taught to you in the most basic of civics classes.