Forcejuggler

I'm a senior and she's a freshman. I don't have much experience with girls because I have focused on my studies and soccer, plus there have been very few girls that I would want to be in a relationship with for various reasons. this girl is great though. she's one of my sister's best friends, a soccer player, intelligent, funny, witty, smart, pretty much my ideal girl. about a month ago, she messaged me on facebook and we started talking a lot. we've both said we like eachother a lot and she said that if I asked her out she'd say yes. I don't know if I want to though. everything about her is great, but the age thing kind of gets to me. any advice?