A.-
First off, sit back for a second and look at what you've accomplished as a person that was too young to be a mother. You've taken something that could have made you just another single mom living off of state welfare and turned it into a single mom that's done wonders with her life. Your drive, your accomplishments and your ambitions are going to be imparted on your daughter. When she's older, she'll be able to see what all Mom did and be proud of you.
I had my first child when I was 21, hubby was 25, we had been married only a little over a year, and we've both always looked young for our age, so I've also received those "She's too young" looks. Stand up straighter, smile at them and go on. DH and I pride ourselves on the fact that we've had children at such a young age because we feel like as younger parents that we're going to be able to enjoy our lives with them more, and when they're older and out on their own, we'll still be young enough to enjoy having time to ourselves again.
As for making friends, age shouldn't be a factor. Find something that you have in common with the other mothers and try to expand on that. If they continue pushing you away, maybe their friendship isn't worth it. If it agitates you that much, tell them straight out "Look, I realize that I'm younger than you are, but our children are friends, and I'd like to get to know you a bit."
Take confidence in yourself and all the things that you've done. Don't go into it feeling like you're "too young to be a mom." Approach it as "you're just another mom."
~S.