Bloody Brenden has been in your exact shoes. He would bring over his girlfriend who was 18 to party here. She seemed cool and pretty hot, but after talking to her for a little I realized why some people have a problem with the age difference thing. Not because it's morally wrong, but just because the maturity level isn't there. And worse yet, they don't get any references to anything. SHE DIDN'T KNOW SALUTE YOUR SHORTS! Brenden recently broke up with her over the maturity thing.
Anyways, I'd say enjoy the relationship while you can. Soon enough she is gonna let her age show and you are gonna get real sick of it. Girls that age don't know how to process jealousy either.
She doesn't know references to many movies or tv shows, but it's not because she's 18, it's because she was limited with TV and all of that shit growing up and mostly played sports. Newer stuff she knows about, but not much older stuff. The thing is, I was also going for this 20 year old chick, and she didn't know who Howard Stern was or 99% of the tv shows/movies/music I ever mentioned. I asked her if she wanted to read my copy of Adam Carolla's book and she never heard of him.
Even though this 18 year old doesn't know much shit I talk about, she makes an effort to find out the stuff. Yesterday she texted me asking "Can I download those shows?" because I had an episode of WM on in the car and she was laughing her ass off.
The jealousy thing, yeah, I can understand that with younger girls. The last girl I was with was 19 and in college and holy shit did she have a jealousy problem. The one i'm with now, she's cool and isn't a psychopath. I know the signs though from the last girl, so the second it shows, I'm out of here.