I married someone 5 years older, so I put +5. But, to be honest, as long as they are somewhere AROUND my age, I don't care. I think having common interests and attitude is more important.
Hubby and I are both atheists, both Democrats, both sci geeks, movie buffs, mystery lovers and like board games. Neither of us drink, smoke or gamble.
Where we differ is that he is an IT guy and eats, sleeps and breathes computers, gadgets and all sorts of machines. I just surf the net. I like board games, he LIVES for board games. I LOVE animals, he tolerates them. He loves Babylon 5 and I love Blake's 7.
Being able to enjoy the same types of things is very important. I'm a fairly unattractive person, so I know he didn't fall in love with me for my looks. We have common interests and approaches. He's a little older than I am, but are experiences have been similar.
I'm not sure that someone 20 years younger than I would really have the same attitudes or frame of reference that I have; I think it would harder to find common ground. However, it's not impossible, not by any means. I just don't think it's as likely.