Organized sports are usually age-grouped until at least 7th-8th grade, so it won't be a problem until then.

In my neck of the woods, there's a mix of middle schools (6-8th) and junior highs (7-9th) that play in the same league, so there's an age cap on competition to keep things fair. In such an arrangement, your accelerated DS could be the only 9th grader still eligible to play.