Scientific consensus is that in many cases, pre puberty transition is best. Scientific consensus is how science works.100% of them... Or.. one cheery picked?
There are specific puberty blocker drugs and so forth that stop a woman (born a boy) from the testosterone surge and developing a beard, body hair, muscle mass, thicker bones etc during puberty.
In addition to blocking testosterone, they can give estrogen to encourage growing natural breasts etc.
The other option is let the kid go through puberty as a boy, then try and undo the changes of puberty.
Physcially and psychologically, assiting a transgender kid through puberty as the gender they identify with is much better for the kids mental health, and cuts out a lot of problems later on.
Depends on the child. They dont just start pumping hormone blockers into a boy that gets caught playing with Barbie dolls. There is a fair bit more to it than that.