1. Make her guess. Make it a game out of it. You can elaborate it into a conversation. She guesses too young, you can say thank you. She guesses too old you can act insulted. Laugh it off and elaborate on her answer.

  2. Make the joke "I'm not sure, it changes every year."

  3. Most important, especially if she appears much younger or older than you: before you reveal your age, ask her "What is your age limit?". Have her establish this. This is one of the first rules in negotiation: make your peer reveal their hand before you do and you gain a serious advantage. Usually she will be generous with her age limit, saying a range that encompasses your current age.

  4. Don't be a dick and don't make it look like you are ashamed of your age or you are trying to hide something. Push her buttons a little bit, but eventually reveal your age. Make her work for your answer, sure, but don't just never give it.

  5. Another option is to take her question and exaggerate it to the extreme. Say some bs like, "I look much younger than I am. I am actually 102. I'm Benjamin Button if you haven't noticed..." or the other extreme "yeah, I'm 12 years old. what are you doing flirting with a 12 year old??? pervert..."

Conclusion: the age question is a common question you will encounter. Be prepared to handle it appropriately.