This is interesting enough to warrant a separate thread, but since we're already here...
I think a lot of it comes down to experience, not just age. My husband and I managed to sow all the oats in our late teen years, so by the time we were 21 (we were both 21 when we got married) we were ready to settle down. Maybe not settle down, but move on to the next big adventure: Real Life Adulthood. We've been married 15 years now and it just keeps getting better.
I have a sister who married a man 12 years her senior. She's very grounded, kind of wise beyond her years, and he's a bit flighty, so in a sense they compliment each other well. Like my husband and I (me? lol) they share the same values and beliefs. Like us, they have been happily married. Seventeen years, I believe.
My brother married a gal 12 years his junior and it was a disaster. As much as my family would like to believe that their marriage fell apart because of his wife's immaturity, I think the fact that my brother is an honest-to-God pig had a lot to do with it. They didn't share values or beliefs. They shared a lack of values and beliefs. Nightmare.