I ride with an eight year old who is stronger and more athletic than most boys his size. But, there is a limit to how far he enjoys riding. He likes to ride fast, but rides fast until he is out of steam. About an hour of riding is his limit. After that, he begins to get tired and cranky.
He rides one speed BMX bikes, and they are well suited to his aggressive riding style. In my very flat town, the advantages of adding 21 speeds seem doubtful.
I'm thinking of getting one of those "hitch-on" type bikes for longer rides. That way, he can work hard until he gets tired, and then just watch the scenery.