I voted -5/+10. I tend to prefer women that are slightly older than I am, and I'm not sure why that is. 25 to 40 is probably a good general range, but there are exceptions. The last girl I asked out is 22.
I'm involved with one person right now, and she is four years older than I am. (I'm 28, she's 32,) that suits me pretty well. I'm not sure what she thinks. Actually, I don't think she knows how old I am... and the night I met her she commented that she doesn't like dating younger men. I'm afraid to tell her now. Luckily I impressed the hell out of her so maybe she'll overlook it.