They want to challenge notions of our current understanding yet don't realize that all discrepancies between how we do things now and did them in the past was born out of challenging our understanding of things. Society only regresses when changes are made in the name of religion or a cult. When changes are made for the betterment of society, such as recognizing that the vast majority of children are stupid because, well, children are fucking stupid! then that is usually not a bad thing. We might challenge whether age is the end all measure of maturity, I know I've met some teenagers more responsible than college students and even some graduates, but age is generally a good starting point if you can't gather much qualitative information. Without meeting someone or hearing quite a bit about them, age is the only metric we can use and quantify that could lead you to a general conclusion about maturity.

And when it comes to "immutable" things, we need to wait until the body and mind have developed at least before we try to change them. Someone's bone structure is not done developing until past 18 years of age, someone's mind only really finishes developing... in the late 20s I think? Early 20s? 100% after they're a 12 year old who tried on daddy's boots or mommy's heels and pretended to be the other gender because they have no idea how any of that works or that they don't need to be a woman to like heels or a man that likes boots!

100% of child diagnoses of being trans are born of stereotypes and gender roles invented by society. Therefore transness can only be as strong as those stereotypes and roles.