I dated a man 17 years my senior for 5 years; we started dating when I was a couple months shy of 20. In many ways we were very compatible. At the time we started dating, I was in the stage of "all the guys my age are immature jerks" and "older guys are super sexy". Heck, I'm still in the latter half of that stage.
But, the relationship imploded largely because I finally faced up to the fact that he really was quite a bit of an ******* and that I didn't want to marry one.
I am now dating someone my own age-- we are only 2 months apart-- and I must say, I really am enjoying this. It is nice to be with someone who has the same growing up stories and gets the same cultural references that you do. It's nice that our friends are the same age. It is nice that I don't feel rushed to have kids or make other big life changes, because the clock is ticking. These weren't things that broke my relationship with the older man. But they are nice things to have, that I didn't know were missing.
Anyway, while dating my older guy, I was a strong advocate of "age doesn't matter!" I'm admittedly a bit more gun shy on the whole idea now, and I admittedly wince a little at the thought.
At this point in my life, I don't think I'd go younger than a year or so less than my age, and no older than 5 years above my age.
But then again, I'm pretty happy with my current dude, so maybe I won't have to ever put this into practice.