My DH is 9 years younger than me as well. We met when he was 18 (our offices worked closely together, in the Air Force), started hanging out together a couple of years later, and married when he was 22 & I 31. I really wasn't sure if I was yet ready for marriage; he was absoutely sure. He's far more mature (wise) than me and most people I know, and he's been a great teacher/friend. We are now in our 11th year of marriage, our friendship deepening. We've been told that he looks older. I tease him about when I joined the Air Force (at 25)he was a high school sophomore!
It's not about age, it's that our personalities are so in sync with each other.