I'm five years older than my husband and I know several people who are 10 years older than their husbands and we have no big issues. For fun, sure! However, I seriously don't think I'd go any further than 10 years apart for a long term relationship.
My grandmother was 12 years younger than my grandfather and he started having issues with senility at about 70 and died at 92. That meant she was still in her 50's and 60's and her life centered around him aging. Now I look at my parents and their siblings and tease my husband that's what he has to look forward to - taking care of me. It's sort of a joke with us, but I do worry if I start to age prematurely that it will be more difficult on him because he is younger.
Another issue is our looks. We looked about the same age when we married -we both looked younger than we were. Now that I am in my late 40's and actually look my age while dh still looks like he hasn't aged, I'm having to come to terms with the fact that he looks quite a bit younger than I do. It's a bit of an ego blow to feel like people are confused about your relationship or thinking he might be your child.