Well, my dad was 13 years older than my mom and that wasn't an issue for them. I'd probably say 15 years tops, beyond that you're getting to almost a generation in age difference. I've been out of the dating market for 30 years, and never dated anyone more than a few MONTHS difference in age.
Only comparison I can make is at work.......anyone more than 20 years younger than me I find I often can not even relate to as a co-worker, I can't imagine being in a relationship with them. They just lack life experience, and are still learning the life lessons I learned 20 or 30 years ago*. As for someone 20 years older, again using the coworker example, I can actual relate to them, but since I'm 53 there is only 1 coworker 20 years older than me, and that person and I have worked together for 33 years and remembers when I was a 20 year old know nothing.
* Example: the co-worker who is 30 years younger than me, living paycheck to paycheck and not understanding why she has no money. Yet she doesn't think twice about getting an i-Phone 4, a $79 a month phone/data plan, a $2,500 HDTV with $199 a month in HD cable service, a $30,000 car, a top of the line Apple $2,500 computer with a $50 a month broadband hookup.........what part of "you can't afford this" doesn't she get?