"My ADHD son always opts for friends who are a few years younger. Should I stop letting him choose his own playmates?"
Your situation is not unusual; many times children with AD/HD 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.