For me I guess I set the ceiling based on my parents age. I couldn't imagine dating someone my mom's age. However, my fiance is 14 years older than me and only 4 years younger than my mom. The age differences was a stumbling block initially, though when we first met we both thought the other was 30
. That was probably the only time when I was in Japan that I thought it was okay to be mistaken as older than I was. The biggest issue with our age difference wasn't our interests and life experience, for us there aren't any, but the reality of what happens as we get older. In ten years I will be 35 and she will be pushing 50, hopefully with an 8 or 9 year old kid. When that kid goes on to University my wife would be 58-60 and I will be around 45. Our bodies will physically be at very different points. This actually caused us to break up a couple times before we decided that the physical wasn't important. Right now we are at the same point in our lives as far as desire, aspirations, and with similar experiences and we will continue to be if we stay together(as much as a married couple can).
As far as someone younger... I've found that anyone younger than 22 is just too young. Most girls, and guys for that matter, under 22 don't really have a cohesive idea of what they want to do with their future. I met a couple 19 and 20 year old girls that were great people and fun to be around. But for me there couldn't have been any future with them because they weren't ready to start a future.