Uh, most people other than myself seem to recount having known since they were a small child. Me, on the other hand, I just felt odd and fake and shallow my entire life while I played at being the perfect girl. My entire identity was built on me being exactly what I was supposed to be, based on my observations of girls and girl stereotypes. That continued until I was 14, when I role-played as a guy online to amuse some friends. Funny thing was, playing him felt more like being me than being me ever had, and it took all of a week to get completely addicted to it, maybe less. There was just something so deep and true and accurate about who he was as a character, that I had no real questions about it: I had to keep what I had in him. Over time, I sorted through what exactly it was that hit me so well, and eventually came to acknowledge that it was, in fact, solely his masculinity that did it for me. A while later, I learned the words "transgender" and "transsexual," and thus began to build the identity including those specific words.
I'm 18 now, and on T. Let me just say that coming out as transgender makes for no obligations whatsoever. There are a million types of transgender: transsexual, genderfluid, androgyne, etc. In any case, coming out as transsexual, even though it's highly specific, doesn't require you to transition right away, or even at all. If you can't afford it, or have medical reasons to not do it, or just aren't comfortable being operated on surgically, you don't have to transition. If anyone says you do, they're dead wrong. You are the one who needs to decide what's right for you, both now and later on. I'm transitioning not because it's expected of me as a transman, but rather because I can't tolerate my body as it is, so whatever it takes, I'm doing it as soon as I'm financially able. If you can tolerate your body, you don't have to do it. In fact, if you're okay with your body, DON'T.
If you know so many transpeople, I would suggest talking to them about it. They'll be able to help you out with your specific situation a lot better than we can. However, if you'd rather talk to me, then you can email me through Yahoo (click my name, click "email"), and I'd be happy to discuss it with you.