I don't know how far along the spectrum your difficult child is. I have a friend who is 18 and learned to drive at 16. He is VERY stereotypically Aspie--the stare, the accent, ripping up paper, etc. And he is a good driver. He never speeds. 
He weaned himself off of his stimulants (don't know what else he was on) and is completely medication-free.
My son is more argumentative than this kid is, and thinks he knows it all. That is problematic.
Other than that, he will follow the rules of the road. But at this point, as a pre-teen, he is still very ADHD.
Still, I'm looking forward to teaching him to drive. He has watched his older sister learn, and has heard us repeat, "Early permit, late license. The more practice, the better."
So I think he's going to be okay.
Don't know if that helps you at all ... I think you'll get a variety of opinions here!
