[Sidhe.]

First: it depends. How close are you to this girl? What do you think she'll say if you tell her? You have to figure out how much you have to lose/gain by telling her (and how much you're willing to risk), and what exactly you're hoping to achieve.
I've been there, not knowing whether to tell someone or not. In the end I did, and kind of regretted it, but different people, different situation, so who knows how it'll go?

Second: I'm not too sure of the age gap there (being from the Land Down Under lol), but I think that if you really like her, you should tell her; and chances are if you're both right for each other, the age won't make a difference. Some people might make a fuss about it, sure, but hey, it's your decision, right?

Good luck, with whatever you choose to do.