This is a great question.
I used to think that age didn't matter, as I've dated guys that are younger than me. My longest relationships (2 years and 3.5 years) have been with guys that are within two years of my age, and my shortest ones (a month to a few months) were all with guys that were over 8 years younger than me.
I've really taken a look at myself lately, and I think for me it's the dichotomy of what I like to do for fun vs. where I'm at in life. When it comes to things I really enjoy doing for fun, it's all about rollercoasters, beaches, anime/superheroes, driving around in the car singing as loud/offkey as possible, and generally being a big kid (the last guy I dated and I bought kites for fuck's sake -- and it was MY idea).
When it comes to life, I've been with the same company for 9 years (since I graduated college) -- I own my townhome, my car's almost paid off, and I work from home. What I really want more than anything is a long term relationship (I'm not saying I have to have one to be happy.. I'm happy being single.. I mean ultimately), and honestly I don't think many guys that are younger are looking for that (or know what that means). Granted, it depends on the person. But my experience says that it's not a good idea to look for forever in someone a lot younger (but that's just me).
There's always going to be parts of someone younger that you can't relate to (because it's been so long since you've had to deal with shit that they're going through), and a part of them will never be able to understand who you are/what you need because they haven't had the chance to experience all the things you have. Like I said, it depends on the person, but generally speaking I've found this true.