I think KY has an age limit for HS athletes. I know a lot of kids were held back to help them advance athletically, but a lot of them were held back in grade school or very early in their development. I wouldn't want to be a kid held back in HS unless I was changing schools, otherwise, you end up putting them behind your peers and friends who are moving on in grade.
I think an age limit is pretty normal, when I was still working, in Georgia you could not turn 20 before May 1 of that particular school year. So some late spring activities had a few 20 year Olds.