There are plenty of CLUB extracurriculars where this kind of thing is a problem.
DD automatically lost two full years of 4H eligibility, in fact, via her acceleration. This is kind of an issue since it's both physical and mental-- so she is definitely competitive with her peers (as opposed to age-mates), but they all had eligibility as 4th-6th graders, and she didn't. (She wasn't old enough then, if that makes sense.)
I seem to recall hearing from another parent that Lego Robotics is another thing where this can be problematic. University, museum, and library-sponsored youth programs may also have (occasionally hidden) policies re: age minimums. When those are because of insurance underwriting, there may be little wiggle room.
In other words, there are quite a few things we've found where there are age minimums AND grade maximums at work. In those instances, yes, acceleration is trading away years of eligibility.
We still feel that in DD's case, it was not a factor in the decision to accelerate, but it is something to be aware of.
