It starts in womb and strengtened at around age of 3-5 years. Skeletal structure after that is not really changing. Maybe you mean bone density? As transwomen starting to lack calcium and other things in bones, so their bones becoming fragile (they are still never becoming same structure as female bones). What I am talking about is the structure itself. Pelvis, for example, joints - they are placed differently and work differently. Male joints are almost straight without and angle, while female joints are placed with angle (especially it is seen around pelvis area), same with muscles and inner organs (different length and structure of colon, for example, no amount of hormones is changing that).

children get access to full HRT.

And now how you will get who is trans and not in womb or in age of 3-5 years, when they do not even know what is social construct. I am a male, not trans, and I played with dolls whole my childhood and still collecting dolls. Should I be transed in childhood for me to suffer whole life after that? Just because "he was playing with dolls and liked pink when he was 3 years old" sexist stereotype?

Editing your messages after they already get an answer is not very nice.

HRT will make you more female biologically

How it will make a nale produce big immobile gametes? How it will change male body to be more suitable for producing big immobile gametes? And even if "sex is a spectrum" (not sure how, as all multi-cell organisms have only 2 sexes in nature), then "80% male and 20% female" is not making someone female.

And as one woman said in other topic:

if you join feminists in USA and they will start speaking about tampon tax - you would not see this problem same as them. Or if someone threatens to rape you or even do this - it wil be horrible, but you will never be able to get pregnant, so fear of rape will be different from your perspective. If you will start smocking or drinking, you will never have concern "and what about my future kid I will born?" as one of negative aspects of it. And many small things like that. Obviously majority of things you will see absolutely from same point of view as we do, but a lot of small things or some angles of view would be still different.