I've lived with this problem my whole life. I can't even tell you how many relationships I've had where the girl found out elsewhere, not from me. For me, it was GAME OVER. Even if she was still interested. But the older I got, the more I relized "9 out of 10 times they want to be with me anyway, so what's the problem?".
I came to terms with the problem was ME. I wasn't comfortable being true to MYSELF, therefor definitely wasn't comfortable coming out and telling them straight out I was transgendered.
Plus, it's easier to "get away with it" at a young age, when you're 14-15 and look like a 14-15 yr old boy. I'm 23, and have heard I look anywhere from 16-23. Depends on the person, and how you carry yourself.
As far as "when to tell", I don't think you need to tell anyone straight off the bat. Give them a chance to get to know you, before you give them the chance to judge you. That way, when you do tell them, if they decide not to talk to you, you know it's because you are indeed trans, not because you're not a good person.
I still to this day have friends from 4-5 years back, that don't know. In the past year I've also came clean to some close friends, that wouldn't of even guessed it. One said "I thought you were going to say you were gay, but that didn't make sense because you're always talking about girls".
Just be confident in yourself. I think the only time it's really NEEDED to tell someone, is when a romantic interest starts to develop.