knowing is a particular thing
is knowledge innate? maybe some is, while most aren't (think instincts, drives, basic motor skills, etc.)
some trans people "knew" when they were little - usually quantified by their subconscious and conscious desires
some only knew until they were much older
if the information is not easy to come across/commonly known and you don't actively seek it out, then the discovery will come at its own pace, often times too late
kids nowadays are more fortunate in that there's so much more visibility and representation and information so readily available out there that it isn't much of a gradual process anymore
they don't have to go through all the pain and confusion of not knowing and just go straight to the pain of knowing