Her parents are just being dicks. They don't want their daughter dating YOU. You're one year older, dude, it's NOT about your age. Whatever they don't like about you, it's something about you personally. Really, where parents are concerned, they don't need a reason, just a "bad vibe" that they get. Maybe they think the extra year makes some sort of difference, I dunno, but it's not the deciding factor.
If you've read my site for a while (or read enough of it), you've surely heard me mention my young friend, Meg. She was 6 when I met her and in the 3rd grade when I started this site. Currently, she's 14 and a freshman in high school. She's also dating an older guy (I believe he's 16 by now, iirc, or almost 16) and has been with him for 8 months at the time of this writing. So, when thinking of a response to this, I asked myself how I felt about Meg's boyfriend. I mean, she was always like a daughter to me and here she is, growing up and dating an older guy. Do I dislike this guy who I don't even know? Do I think he's only after sex? Do I want to smash his face in with a brick? The answers are no, maybe, and no. Maybe he IS only after sex, but if that's so, then he's either awfully committed to it or really fucking desperate since he's been hanging in there for 8 months (I'm assuming she hasn't slept with him, of course, but even if she has, he's still with her so he obviously wasn't after JUST that and the point is irrelevant).
In my little thinking-out-loud session about Meg, I pulled your answer right out of my ass. Clean it off before you use it. Time is the key. Based on the fact that Meg and her boyfriend have been together so long, I don't have to worry that he's only with her for sex; and even if he was to start with, he isn't now. And that's what it will take for YOU as well. Just time. As time goes by, they will come to trust and respect you more and more. Without ever having met him, I like Meg's boyfriend very much, simply for the fact that he obviously makes her happy. Your girlfriend's parents will reach this point too. It may take longer than you want it to, but if you stick with it (and they don't absolutely forbid her from seeing you), it'll happen.
If they do (or have) absolutely forbid you from seeing her, then my advice is totally different. Fuck those assholes; keep seeing her behind their backs. It's not so hard. I dated a girl for nearly 2 years behind my parents' backs. That made it kind of fun, actually!
If you are after her just for sex, go someplace else cuz this is going to require more of an investment than it's worth just for a piece of ass. It would be like spending years of your life and thousands of dollars going to culinary school just so you can make your own goddamn Twinkies instead of buying one.
All My Hate,