My son is accelerated by grade and is with his classmates, not age mates, for Cub Scouts. The funny part was when he joined in first grade as a Tiger Cub the main office for our scouting district had to "fudge" his birthdate because of some computer program that wouldn't allow his birth year for a Tiger Cub that year. I guess it automatically assumed that all kids in scouts were a certain age and he wasn't 7 yet! But now they have supposedly corrected it (we just haven't had any paperwork to confirm the correction).
MrWiggly has had no problems being with his classmates in Scouts. I don't anticipate a problem crossing over to Boy Scouts, either. But his acceleration was a really good fit, so maybe that helped.