One can also seek out mixed-age environments that focus on achievement. Martial arts classes and boy scouts are both like this: various ages are interacting, and the kid progresses on the basis of fulfilling requirements, not on the basis of age.
We also get great mileage out of camps that serve an age range-- always registering our kid when kid will be the youngest kid in the range. By the end, seeing kid's participation, the teachers are well convinced, and that also boosts the kid's confidence for the next time.
