I believe I read that a student athlete cannot have turned 20yo at a NEPSAC school. I’ve known PGs who turned 20 shortly after their season.
The only other downside, is how a student feels about being older than the majority of their class and/or graduating later than friends at home. An extra year of HS may not appeal to many!
I think the reclassed 9th graders feel “old” that first year in relation to their classmates. But that fades by 10th forward.