In one of my high schools there was a 25 year old retarded kid that had his license. At least he said he did, and we saw him driving a few times. He used to have "parties" in the back of his car...which was odd...he'd ask everyone he saw in the hallways if they wanted to party with him in his car. When they'd say no, he'd ask if they needed a ride home right then, regardless of what time of day it was.
So yea, apparently retarded people can eventually drive. In NJ at least (big surprise there).