Personally, I think a 20-year age difference is too much. What happens when you're 50 and the other person is 70? There's quite a large gap there as far as things go physically and perhaps even mentally to some degree.
I wouldn't mind a 10-year difference, but even that is stretching the limits for me a bit.
I also totally realize that it's not about age and looks, but yes, many times it is. You have to be physically attracted to the other person. You simply cannot make things work when you aren't physically attracted to a person. I know I sound really shallow, but that is just the way it is.
I think the limit for me would be 5-7 years younger or older. That is my final answer