It does seem kind of insulting to the transgender individuals who didn't act until later in life to insist that someone can't be confused about if they are tg or not and that it's obvious.
Could be. I'm not really interested in whether something is insulting, though. Chaz Bono exists, whether or not he is insulted or whether people are insulted on his behalf.
There's also a problem here when listening to adult transgendered individuals talking about their youth, because by the time they get to be unambiguous adults, they know. (Some might disagree, but I don't.) This doesn't help the other way around: how many diagnosed transgender children will grow up to be transgendered?
However, I reject that one can't know gender until one has known sexual interaction. That premise presupposes that sexuality is the overwhelming component of gender. It isn't for everyone.
You can disagree if you like, but your supposition of the presupposition is fallacious. The presupposition is that maturity and a variety of life experiences are important for understanding gender.
Now, I happen to think that sexuality, in the broad sense, is an overwhelming component of everything, including the drive behind intellectual pursuits and learning, and emotional maturity. Even male dogs, who have notoriously simply sexualities and intellect, fly the pink zeppelin when humans teach them behaviors.
This is a minority opinion, but if you balk at that, consider this. Our culture does not allow pre-pubescents even to make decisions regarding their sexuality, or refuse consent to medical treatment, or enter into binding treatments. So, I'm not convinced that it is possible to get good enough readings from them to warrant the Harry Benjamin protocols involving hormone treatments of pre-pubescents.
Even if you balk at that, fine! Let's discuss it as adults. It would be a couple of orders of magnitude better than what has happened in this thread, or any other I've seen.