There has to be a lower bound, but above that it's just a number and case-by-case to me. I've known 56 year olds more immature than some 18 year olds.
I'm about 30, I've been in relations with 3, 13, and 31 year age gaps, myself always being the younger party. Being on the other side of all those I personally don't think I can generalize any other rules for other couples. Just some minimum for protecting vulnerable people.