It's a classic sh!t test or a congruency test. She's not going to test you with lines like "you're too old" if she has absolutely zero interest in you; she would just ignore you instead if that were the case.
@Ricky and
@Slag offered humorous answers that demonstrate that it doesn't bother you, and the key to passing a sh!t test/congruency test is showing that her tests don't affect your frame or throw you off guard.
Another classic formula is to agree and exaggerate. Agree that you're too old for her and pretend like you're an ancient grandpa. Agreeing makes sure you're not being defensive (showing insecurity) and exaggerating adds humor to make her potential objection a non-issue.
Some people say that you should never bring up age, but using age to say that she's probably too young for you can be a good tease at times. Before she ever gets the chance to tell you "You're almost double my age," imagine if you had said "You're almost half my age" to her instead. With some people, this sort of challenge will instinctively make them want to prove you wrong, that they're not too young and age doesn't matter to them. There's nothing more enticing than telling someone they can't have something.