I agree, it would be more enforceable.
But I think my question is whether we would really want that. Should a kid who gets held back for academic reasons, and then gets himself back to the point where he is doing ok be ineligible to play sports because he got held back at a younger age (which if we're being honest usually means he didn't have the support at home, and is really the victim here)? To what end?
And on the "age and physical size, for player safety" piece, I don't even think age is the primary determinant of size by the time you're in HS. Certainly not to the degree that one additional year of age makes a huge safety difference. Otherwise you wouldn't let Juniors play with Seniors, or Sophomores play with Juniors, and so on. And if it's just a safety thing, then you'd have kids "size out" instead of "age out." "Sorry Sophomore Kofi, you're too big, you're going to have to sit out the next two years. Good thing AAU basketball exists"