I was around my early teens (~14-15?) when I thought maybe I was a ftm, I had just heard of the concept from livejournal (where ftm outnumbered cis men easily), was fully sexually imprinted on yaoi stuff and fantasized almost entirely on m/m (m/f grossed me out), and was already a bit of a tomboy (too awkward for nice clothes or makeup).
But the time was never right to "come out" and I think I grew out of that when I hit adulthood, by the time I was 19 I was just a tomboyish-identified bisexual girl and by my mid-20s I was downright feminine. RN I love being a woman minus whenever I get my period. So for me, I think it was just temporary awkward tomboy confusion, that kinda sorted itself out as I got older. Everyone eventually settles down where they will, and if they shift around in their identity so what?