Delay as long as possible. Every child is different but be prepared for accidents. This age is cocky to begin with but if they don't have the maturity yet than they could hurt someone else and you will be legally responsible!
Be prepared that a high functioning Aspergers child or high functioning ASD child might not be able to drive ever because they have delayed response to things and it could be highly dangerous, as a car could be a deadly weapon. The Dept. of motor vehicles or your school drivers education would be a better judge of this. We as parents sometimes want our children to be able to do everything but we don't own up to that "this is not a reality".

My son wih Aspergers got his permit at the same age as his peers. His dad let him drive our truck on their family property. He had a dirt bike at an early age. He was afraid of it at first. Then asked if he could race motocross. He went down a lot initially. However he learned balance and got to be a great rider. His moto friends overlooked the things other kids made fun of him for. We always let him know how proud we were no matter where he placed. He eventually started placing better and better. He has big trophies to show for it along with professional track photos we blew up into poster size. He is a better driver than we are. He even backs up a truck with the trailer like a pro!!