Hi Terese,
I'm sorry to hear that you and your son experienced this teasing. Have you checked out Education.com's special edition on bullying and teasing?
http://www.education.com/topic/school-bullying-teasing/
This article in particular offers some concrete actions you can take:
http://www.education.com/reference/article/parents-witness-bullying/
If the students who are teasing your son are in his classroom, I would speak to his teacher about the issue. If the students are in another class, approach the principal for follow-up. Regardless, you shouldn't stop being involved in your son's school (or stop walking him to class) because of this.
After explaining to your son that the teasing behavior is unacceptable, you might also point out some of the benefits of having a young, vibrant mother who loves him very much. I wouldn't go so far as to put down mothers who wait until later in life to conceive, but you could emphasize that he's fortunate to have a mom who cares enough to be involved in his life, to take him to class, to feed and clothe him, and love him unconditionally. This is also an opportunity to talk about how not all mothers look alike, and that it isn't as important how you look as how you treat people (including those you love).
Good luck -- I hope you are able to both put an end to the teasing, and to strengthen the bond with your son from the experience.
P.S. For what it's worth: I too have a seven year old and look young for my age. I also grew up with a mother who was less than 20 years older than me. While we haven't yet experienced the teasing you did, I do empathize with you, and wish you the best in resolving the issue.