“My son is almost eight and enjoys playing with children about three years younger than himself. He will play with children his own age only when it is one-on-one, but given a choice, he would pick his younger cousin to play with. He is extremely bright, but very easily frustrated. Should I sit back and let him choose his own playmates or encourage him to move on a bit?”
Your situation is not unusual; many times children with ADHD take a little longer to develop some of the social skills needed to interact successfully with peers their own age. It sounds like his interests are more similar to that of younger children. I would not recommend forcing him to play only with children his own age. He may also be able to give you a better understanding of what he likes better about younger children.
Watch him interacting with peers his own age and try to determine what skills, if any, he is lacking. If needed, help teach him any missing or limited skills so that he can be more successful with peers his own age while keeping his self esteem intact.