It’s really intriguing to think about. As Camokat mentioned upthread there are differences between the sexes prior to puberty so I imagine there’d be a longstanding advantage there. Puberty blockers are only recommended children take for around four years due to their effects on bone growth.
I guess the other thing is that it would super easy for that trans person to stop or reduce cross-sex hormones, get the benefit of increased testosterone during training, then recommence hormone treatment sometime prior to competing. As nice as it would be to assume no one would do that, we know that people do all kinds of insane things to win at sport and this would be an almost indetectable forum of doping, I think. Particularly because the testoterone requirments for transwomen in elite sport are much higher than the majority of women produce.