I agree with what all of the girls have said. Ignore them. Or if they are just getting to you too much, tell them off. I have little patience for people who stick their nose in other peoples business, only to make that person feel bad. I would tell the person, Yes I might be young, having a baby, but I'm not the first person who's done it, I sure wont be the last, but at least I'm being responsible and owning up to my actions, and taking care of myself and my child. You are being more mature just by taking care of your baby, than they are by criticizing you. I'm 20 & DH is 21, and occasionally we get a look over, but no one has had the nerve to say anything to us. Then again, I think, er like to think that I look older than 20. I've been going out to clubs since 18, and its a rare occasion that I get ID'd. (Drinking age here is 21, Had a fake, thats useless now LOL).
Anyway, blow em off. They are probably people you will never see again, and just know that you are being an adult by taking care of yourself and lil bubs. =)