When my son was 8 or 9, 10 to 12 miles was about the limit. By age 10, or 11, he got a lot stronger, and he did a 25 mile ride 2 seasons ago, though I didn't tell him in advance how far we would be going. Last season at age 12, he rode many 20 to 40 mile rides with me on an old Trek 800 mountain bike. Maybe this spring or summer now that he is riding a hybrid, it is time to try a multi day ride.