I fit the stereotypical 'child that knew' narrative sufficiently that gatekeepers have effectively rubber-stamped and waved me through thus far. I appreciate I'm incredibly fortunate for that which is why I loathe the narrative imposed on trans people and the way it's used to judge legitimacy. I don't think anyone should have to doubt their personal experience due to unwarranted expectations or fit through very small hoops in order to justify their unique identity. It creates arbitrary and detestable suffering for so many.
For me, the narrative seems to be a product of trying to survive in a gender essentialist society that's determined to protect its weak foundations. I'd guess that if we offer definitive conclusions that fit a predictable pattern then we become a somewhat known quantity and less of threat to dominant ideas. It ensures as many trans people as possible can be assimilated into the existing prevalent gender model and limits confrontation with the authority of cis assumptions. Those that have power over us are prevented from questioning their world-view and maintain the illusion of definitive categories. Girls remain sugar, spice, and all things nice while boys remain slugs, snails, and puppy dog tails. We're just 'errors' that can be rectified and hidden as often as possible.
It's why I tend to giggle at the accusations that trans people reinforce gender stereotypes. We're not the ones that created those stereotypes and we're not the ones heavily punishing anyone that transgresses them. If trans diversity were actually acknowledged and accepted by professionals and society then I'd guess, and hope, that the restrictive gender essentialist edifice would crumble.
In terms of personal experience with this, a psychotherapist said she was very supportive of me moving forward with treatment due to 'showing a powerful sense of femaleness.' While I was relieved to be getting what I need, it also made me incredibly peeved since explanation quickly revealed she meant my body language, habits, conversational language use, etc conformed with stereotypes of femininity. She used my personal gender expression to judge my core identity and the possibility of her doing that to people whose expression doesn't concord with stereotypes scares me.
Another aspect of the stereotypical narrative I tend to think about is whether causality was reversed for me. Did I absorb the narrative then begin interpreting my life in accordance with it? The fact I first expressed my discomfort and need to be female at age eight would suggest not but I also found information on gender dysphoria when twelve so it's entirely conceivable that the weighting I've given to childhood memories or life experiences since that point has been unwittingly affected by that.
Anyhoo, sorry for rambling. Thank you if you read that nonsense. Hehe.