It kind of annoys me when young girls say "age is nothing but a number", because that simply isn't true. With age (usually) comes experience. People go through different phases during their lives, and when a younger person gets involved with an older person usually they realize this after a while.
My best friend is in her early 20's, and her husband is in his late 30's. They're in marriage counseling now because they've realized exactly how different their plans are. She wants to stay in school, complete her education and establish a career. He's been through school, has a career already, and wants to have children immediately. She's not ready for that, and he's not wanting to wait. Also, he has a tendency to treat her more like a daughter than like a wive, and until recently she has been allowing him to do so.