This is definitely an interesting question... In my case, I certainly agree that if I'd been MTF, things would've been a lot different for me. I've been dressing and largely acting male for years, I never considered myself a "normal girl." It's hard to say for me when I really "realized." I've really been extremely uncomfortable with my bio gender, and particularly with my body since puberty, and spent a long time trying to figure out what exactly was wrong, but it never really occurred to me "oh hey I'm a boy" until I started reading up on FTM websites. (Which I first ran across about a year ago while researching good binding methods, since bandages didn't do much for me. Bit of a tip off, maybe?)
Yeah, I don't know. It was never really a completely certain thing for me, I guess, it was more of a "this makes sense," and then realizing just how much it makes other things make sense in retrospect. One of the big things is how I've always identified way, way more with male characters in books, movies, etc. All the characters I wanted to be just like when I got older: All guys.
Right, that was long and rambling. I'll shut up now.