As a self-admitted dirty old man, here's my take on it: if an 18-year-old guy wants to bend over for me (and believe it or not, there are a few who do), I'm fine with that. And I think older/younger can make for good "friends with benefits": I think the older has much to teach the younger from his life experience; and the younger has much to teach the older from his fresher point of view.
However - for a primary love relationship, I think it's best for the two partners to be in the same stage of life. If one partner is finishing college and thinking about what his career might be, and the other is contemplating retirement, it's going to be difficult for them to row the same boat, as far as making a life together.
PS: My husband and I are within 3 years of age of one another, so it's not like I've ever tried a May-September marriage, so what the bleep do I know?