Honestly I believe to every person the answer is different. There are some who would never date someone younger than themselves or older than by a couple of years. Then there are the people who go by looks alone.
Personally I do not wish to date a guy younger than me but I have a rather broader look on how much older they can be.
Personality is the key though, I can't stay with someone who can't draw emotion from me. So personality can get you farther than looks any day. Like people have said, in the end looks fade, people get old, but if you have the personality going for you then you can survive well.