Think of it this way: guys like George Clooney and Robert DeNiro are WAY older than you, and although they aren't picking up 18-year-olds, they are getting with hot women that are MUCH younger than them. Nobody thinks they're creepy, they are STUDS
In all seriousness, I've been having a similar issue (although the girl I'm seeing is just 8 years younger), so I think I can relate. In the end, I found that the truly important thing is if there is chemistry. Can I have a good time with this girl or not? Whether or not you are looking for an LTR or just to have some fun, just ask yourself that. Now, you might find that as an 18-year-old, she is too immature for what you're looking for, but hell, it could be a lot of fun as well.