I don't think 10 years is a major difference after maybe age 27 or 28. Before that, it might point to immaturity (or control issues) on the part of the older person. (Disclaimer: I dated older when I was very young, until the guys caught up with me maturity-wise.)
I've always liked guys very close to my own age. I have younger brothers who I used to babysit so I have a hard time going more than a couple years younger - I feel more maternal toward younger guys. I want to fix them up with girls and help them solve their problems.

(I would be a terrible Cougar.) Yet I am kind of "young at heart" in my interests at 40, so older guys are often just too old for me. I'll be excited about the latest Calvin Harris dance song and they're waxing poetic about the Rush concert, and I can't relate.