Joseph – Up to 10 years seems to be a common opinion.
I know a shadchan who seems to consider that a halacha. Whenever I ask her if she can set up two people with each other, if they are more than 10 years apart, she will nix the idea immediately and say “they are more than 10 years apart” as though it’s assur to set up people who are more than 10 years apart.
Personally, when people ask me up to what age I would go out with, I don’t like to give a specific number since I don’t believe in making such hard and fast rules, and I think it depends on many factors. But if I am forced to give an exact number (for example if I am filling out an online form), I usually say up to 10 years older.
I do know girls who married guys who were more than 10 years older than them and they seem to be happy. And I don’t believe in making age too much of a factor (in either direction) unless it really bothers the person.
The only real problem I have with girls going out with guys who are much older than them is that it gives older guys unrealistic expectations and they refuse to go out with any girls who are older than 10 years younger than them (since they know that they can occasionally find girls younger than that to go out with them) Meanwhile, these guys just keep getting older without getting married and there are many girls who are still single because these guys won’t go out with them.