Well, there's the old "half your age plus seven" rule -- but I just take it on a case-by-case basis, really. I've seen more age disparity in my dating life than most people I know who, generally speaking, seem to have stayed within a 3 year age bracket (over or under). In my early 20's, I dated a woman in her early 40's and I'm currently talking to a girl who's more than ten years my junior. This also holds true in my friendships where the ages range from 18 to 75.
The reason for this, I believe, is that age is mostly incidental when you're wired like I am (and most INxx's probably are). I tend to care little about attributes that are contingent upon chronological age such as physical perfection, ability to sustain high energy activities, partying until 4am, overt familiarity with pop culture references, etc. For that matter, I have little regard for orthodox relationship trends set forth by Western culture. What I like has more to do with attributes like intelligence, how she interprets the world, what's going on inside of her mind -- things which are only very loosely tied to her age. I mean, a brilliant 21 or 41 year old female who is well-read is going to be far more attractive to me than a 31 year old female who has a nightly ritual of watching reality TV and passing out drunk.