Them: "You look too young to be married!"
You: "Oh. Well, I am. Now, can you help me find those shoes that we're looking for?"
Really, it's not anyone's business and although I doubt the comment is intended to be unkind, it's still inappropriate to comment on someone else's appearance. By directing her attention back to the task at hand-her JOB-you are subtly letting her know that her comment is unwelcome without being unkind or rude. It's a lesson a lot of people should learn...