I advise staying out of her for as long as you can. The longer you hold off, the more meaningful the sex will be. Aside from that, supposing you go ahead and dive in, and then her parents find out, you're going to have a lot of problems with her family. You said she has the thinking level of a fourteen year old? I've been in the same boat; my ex had some mental issues and had a lot of the aspects of a young child. I'd tough it out til her 18th birthday, if I were you, and ask lots of questions in the meantime. Her feelings on the subject probably aren't cemented in her mind yet. Fish for doubt, because when she's in the mood for it, she probably isn't considering how she's going to feel about it later, and how she feels about it when she's not turned on. Other than that, your first time together would make one hell of a birthday present, so that's another reason to hold off. Masturbate vigorously and make yourself spent before you see her, should you need to.
We don't know your girlfriend. You are the only person of all of us who'd be likely to know what's appropriate for you two as a couple. But that said, this stuff is best left until you're 100% comfortable and 100% sure, and you've already said you're scared, so I'd say not yet. Also FYI sex is massively over-rated.