re: I heard a theory on the Discovery Channel, once
Sexuality.
I'm sure there are odds, but I think there are many contributing factors.
With age comes experience. If women have their peak sexuality between 40 and 50, then the ones in loveless marriages must go mad through that decade.
I don't know where they got that meme that only male teenagers are at their sexual prime. Frankly, I think that with age, comes experience and sexual health is better enjoyed once the pressure is off and partners have a clearer idea of how to satisfy each other.
BTW, most women are shown to have therir first affair at the age of 35 (if they're married). They are entering a cycle ruled by Mars. (Or so they say.) I know I fit into that category along with a couple of my closest friends. How about you?