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.)
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.
Our daughters are in 4H also, and we had one of them start when she was 8...the wording (in OH) has since been changed so that now even if 3rd grade has been completed the child has to be 9.
All of these arbitrary rules are frustrating.
Also, because of a grade skip the same daughter is competing in high school track as a freshman. The average freshman around here is 15, so she's definitely at a disadvantage (she's 13).
There are so many things to weigh in these decisions! If she could take college classes and still play high school sports, that would be ideal.
