In the end most people are cis, and they never spent a second of their life seriously questioning it.
I actually suspect this is something with which many posters here would disagree. I do wonder if it's another key difference between the male and female experiences of growing up.
Many girls are deeply uncomfortable with their bodies in their teenaged years - not just a superficial, Adrian Mole, awkward curiosity, but a visceral unease or even dislike.
And for most of us here, this also came with a deeply-felt sense of being "different" or even "wrong".
My teenaged years were defined by a stoical, troubled resistance to flirting and make-up and dresses and gossip. I detested the way my peers succumbed to it, and found it degrading to be associated with them - found it degrading, at times, to be female, in fact. If "non-binary" had existed then, I'd have clung to it like a deceptive life raft. Deceptive, because it would have confirmed my conviction that I was somehow fundamentally different.
Because I wasn't. It was simply that teen hormones + a highly sexualised society was working its ugly magic on my peers, and I recognised and felt uncomfortable with that. I was no more, or less, female, or girl, than they were.
The thought that I could have been inveigled into the kind of support groups or counselling Aida suggests above, simply as a result of resenting bodily changes that highlighted my physical vulnerability, and feminine stereotyping, is horrifying. No longer been simply an independent thinker, oh, no. Instead, not "matched" with my body. Part of a tiny minority who could benefit from actual therapy because of my entirely natural resistance to the box society's representation of my female body was trying the squeeze me into!
I could very easily say I'm non-binary now. I actually think I fit the definition. Unease with my body in some respects - yes. With gendered roles in many (all?) respects - hell, yes!
If I am non-binary, though, where does that leave everyone else? As a contented, signed-up member of a neat "gender binary"? There's really no such thing. (Except perhaps for a dysphoric trans person, whose resistance to their birth sex is so absolute that the define themselves in opposition to it?)
Anyway, to my mind, to say I'm non-binary would be to uphold an artificial binary that does no one any favours.
It's such a bizarrely absolutist way of thinking. It feels so very reductive and regressive.