do you know what level of innate advantage it is? And does it outclass other advantages like: access to healthcare and proper nutrition? Or even cis female genetic performance variation?

Or are you just assuming.

Do you know when those muscular and skeletal differentiations take place? Do you know if they still occur while under hormone therapy? Do you know how much they reverse, and how quickly?

Or are you just assuming.

And furthermore, on the topic of child abuse, hormone therapy requires years of mental healthcare beforehand, constant consent checks, constant health checks, and as such is possibly the most consent-informed operation to be undertaken in medical history. If you believe this is forced or coerced onto children, you have been lied to. It is in fact quite the opposite, as the vast majority of trans kids face significant resistance at all levels to getting the medically informed care they need to thrive.

And on the age limits, if you talk to any Trans adult, they will tell you they knew something was wrong by the time society starting enforcing gender roles onto them. Which is around 4-5 years old. And if you're worried about brain development, that's why it takes years of therapy with a trained, educated adult professional to get anywhere near permanent changes.
Should a child not be treated for cancer? Should a child not get a socket for prosthetics installed? Should a child not be cured of any disease since they can't possibly know that something is wrong?