Psychologically speaking, and I hope I don't offend you, she could be looking for a father figure, or masculine approval. What's her relationship with her father? If it is a good one, then it could be just one of the situations where age is not a factor--at least in her mind. But if her father is absent, estranged, or their relationship is otherwise bad, I would consider steering clear of a non-platonic relationship. When I was nineteen, I had a few friends (my age) that were going out with much older men--late thirties, early forties--and it was almost always because of a lack of a strong male role model. Girls need their father, and when they don't have him, they often resort to promiscuity and make bad choices in their significant others.
I'm not saying you'd be a bad choice, Drake7 (that you have thought to question this makes her lucky) but it could be a very messy situation for you. If all she wants is a sexual relationship and I were in your shoes, I'd check into her family and then consider it. However, anything more long term, you two may have very different goals. She may want kids--do you? Do you already have them? There's a lot to think about.
Speaking as someone who fits into that 18-24 age gap, those of us that seek out a relationship with this kind of age difference are few and far between, and most do it for the reasons I explained above. Most of us want family and kids, and an older man usually has already been through that, and doesn't want to do it again. You may want to just be there for her as a positive male role model--not necessarily a father figure, but someone who can protect, advise, and support her as a father would. Someone like that would have made a world of difference to the girls I knew a long time ago.
Morrighan