There was a section in my school with mentally challenged kids, and I can say from dealing with them first hand, that many of them aren't responsible for their actions, even when it comes to physical violence that doesn't have to do with rape.
When I was in.. Junior High, I believe, there was this very strong kid with Down Syndrome, who used to start fights with people, by coming up to them and whaling on them, for no reason. On at least two occasions, I was at my locker, minding my own business, and the kid got away from his class, walked up to me, and began pounding on my back, until I was black and blue. I reported him to his teacher, and the principal, to try and get him to stop, all to no avail. One day, the kid and his class had filed into line to go to lunch. He saw me, and tried to hit me, but I blocked his fist with my hand, and whacked him hard, in the stomach. I said to him very loudly "Andy, you have to stop hitting people, it hurts, every time you hit me from now on, I'm going to hit you back!". I dont know if what I said to him went through, but, from that day on, he never hit me again, and he looked a little scared of me when he saw me. I didnt like to have to hit back a mentally challenged kid, but nor did I want to put up with his hammering on my back every day for no reason.
I tell this story because I think the only way the mentally challenged teen would have understood that what he was doing was wrong, would be if someone had physically done the same thing to him, over and over, and said "This is wrong". Mentally challenged people have different levels of understanding, but sometimes, as I had to do with Andy, they had to be shown that "This hurts" in order to get them to stop doing so.
I hope my post isnt off topic. I just wanted to explain my experience with physical violence from a mentally challenged person, to try and put into perspective that many times, they might not well understand what they are doing.
Mitch