This.
To prevent child abuse or child exploitation in "big sport", all athletes should be legal adults. However, the problem is that the age when a person is legally an adult differs from country to country from 16 to 18 to 21 and then there is also "adult enough to" factor, like, adult enough to get married but not adult enough to drink alcohol

Tbh I think that 16 is adult enough for sport (even for women, even in figure skating, even
to be women in figure skating). However, I'd see no problem with 18 years either. Just add another junior category, like, "junior seniors" or "senior juniors"