Did someone ban Digital spy yet? that needed to be done a long time ago.
Death Shredder and Pump have the right idea but let me give you a different spin based on mostly dating girls much younger then me.
You can start by playing the fun guessing game, or any other funny, cocky, clever responses which avoid the question but realize they normally wont work, a girl will just keep pressing until she gets an answer (if you haven't built enough attraction) so I have largely given up trying to be funny/clever with answering the age question. Of course you are best off when you built solid attraction, than they dont even ask before sex
Here is my standard response
"Ill tell you when I see you" If they press me I still dont answer. Most times they will just say "ok

" and take it as a shit test passed. She likely already knows you are significantly older than her and the exact amount wont affect her opinion at this point.
Now here is why I believe this is the best response de facto. If you just tell her your age straight away than she has a legitimate reason to immediately disqualify you, as someone already said, as you haven't likely built enough attraction and you have provided her a rational to ignore you, and chicks love to rationalize. If you dont answer the question and she ignores you, guess what? She was going to blow you off anyway as soon as you told her your age, so you haven't lost anything by being firm. The reason I like my answer better then the guessing game, or cocky-funny responses, is that it comes off as higher value, and it doesnt look like you are trying to avoid the question.
The best situation is you have enough compliance so she doesn't even ask you that question till later.