Your age doesn't really matter if she's interested (and you're legal) but you might want to stall the question a while until you've got a few IOI:s. Here are a few answers:

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Take a step back and give her a slightly confused look.
YOU: "Wow, I never really figured you to be the shallow type."
HER: "I'm not shallow!" / "I just asked for your age!" / "Why do think that's shallow?" / Any other response.
YOU: "Okay, if you're not shallow, why not ask me something a bit more interesting?"

If she asks you something else, you've successfully avoided the subject without appearing to hide anything.
If she insists on getting to know your age, just tell her you're going to look for your friends (or go do whatever) and that she can come find you when she's got a real question ready.

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YOU: "I mostly decide from day to day, right now I'm about sixteen. How old do you think I should be?"

A fun way of pushing the question aside for a moment. Be ready to switch the subject after she answers and you might get away with it completely.

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YOU: "No no no, now you're supposed to ask me [insert any question here]. Questions about age comes in the third stage, is this your first time picking up a guy?"

The last part can be directed at her friends instead, "...is this her first time..."

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Put on a worried face.
YOU: "Do I really look that old/young?"

This won't really let you off the hook, but you've set up the stage nicely for a guilt trip if she comments on you looking older or younger than you are.