When I finally let myself be one. Like, instantly. I was in denial for 12 years, always wondering and cursing why I couldn't be a girl, that wanting to be a girl so bad that it kept you up at night was "a thing that was only for trans women" and completely denying myself the fact that I could just... do it, that I was what I dreamed of being. I flirted with the idea for years and years, and finally, when I let myself just do the tiniest thing and try my new name (which was at the time just a one-n "Lena",) the floodgates opened and I was never looking back just a day later.

Also, I don't know if you're asking this out of curiosity about just myself, or if it's more than that, and I won't presume, but if it IS more than that: You can just be a girl.