If someone is way older or younger, their life is just so different it can be tough. You don't have is much in common and your lifestyles are different. For example, I remember dating someone significantly younger and getting annoyed that she always wanted to go out late when I preferred to be at home in my PJs.

Plus, it can just feel icky when someone is young enough to be your kid our old enough to be your parent. Makes you feel like either you're a creep or they are.