Adults can't compete in kids' categories either, not even in boys' or young men'. What an indignity, to be forced to compete fairly. Womp-womp.
No adults are training their way to the kid-lympics and then getting cut open and surprised by the count of the rings.
Also, the idea of "fairness" is overstated, a naturalist just-so fallacy. Is it "fair" that some male athletes are taller or shorter than others, or have other genetic advantages, for example?