Love or Lust?

I think perhaps I'm starting to fall in love. I'm not certain tho, the girl is so much fun, so bright and bubbly and sexy, and far, far too young for me. She is 17. Maybe I am trying to recapture some of my lost youth, maybe I really am falling in love with her, maybe it's just lust and curiosity. Meeting her didn't make any difference to how I felt, kinda made it worse in a way. I'd never tell her, we flirt in a jokey kind of way, but to tell her I think I'm in love with her would only damage our friendship.