Hmm... well, my wife is 5 years younger than me, but I'm 26, so completely different story. Check the age of consent in your state; in many states, it's 15, in which case you'd have 2 years before you could officially do anything physically. That, of course, doesn't prevent you from having a good platonic relationship with the girl. If you like her that much, be a good friend, and perhaps it will develop into more.
SD