Maybe it's wanting what you can't have. Or proving yourself worthy. Or being the one to change a previously unchangable man. (Men want to save women, women want to change men for the better.) It's always been acceptable (encouraged, required) for young women/girls to marry older men. I'm one of those guilty of enjoying Spring/Fall (or Spring/Winter) romances. The first time I saw My Fair Lady I was smitten with Rex Harrison and my mom made your same point. I think she was a little a little disappointed her daughter would fall for this romantic clap-trap.
The worst example is, I think, Lolita. Yuck.
Little Women (Jo and her husband)
Gigi
Girl With a Pearl Earring
Laura Ingalls and Almonzo Wilder
Rebecca
Jane Eyre
Warning :: Spoiler Ahead! Highlight to read message!
I know, I know, it's icky. But I like the Hermione/Snape fanfiction stories.
I don't know how old the Phantom is supposed to be, but maybe Phantom of the Opera fits here.
================================================== =========
Eta, House and Cameron from tv's House. Though I suppose they're not together. Still, she's attracted to him and he's about 20 y.o. than her.