The Onion

You realize that most trans women undergoing HRT have less testosterone in their systems than many of their cisgender competitors, right? Women naturally have some testosterone in their body; if they have PCOS, then they have a lot (in comparison to other cis women). Trans women on HRT take medicine to block testosterone, so while their body might generate the most of all, the amount bioavailable to them after the medicine neutralizes it is less than non-PCOS cis women have. If the sports debate was about testosterone providing an unfair advantage, then we would ban cis women with PCOS from competing, but we don't.

In the case of teenagers, it's even simpler - contrary to propaganda, very very few doctors are willing to put a minor on HRT. Instead, they simply give them puberty blockers to delay the onset of puberty until they're over 18 and can decide for themselves whether to go on HRT or not. So, a 17yo trans girl is developmentally equivalent to a 10yo boy - her muscles are massively under developed compared to any of her cis competitors.

So whether adult or teen, a trans woman winning at sports is not someone with an unfair advantage; rather, it's someone with a massive disadvantage managing to win despite her handicap.