I totally agree with the "too young" thing. I'm 15 and been freelancing since I was 12, and this gets in the way all the time. People try to take advantage of you once they realize that you're young(er) than most in the field.
I would agree that you should be paid based on your skill, though, not age.
I'm not trying to push you down, and anyone not paying you what your work is worth is definitely taking advantage of you.
Well it does, but you obviously benefit from all relevant experiences. Any kind of design, even print or illustration, is relevant. Obviously all programming, not only PHP/Javascript is relevant.