Artemis,
I think it's more biological than societal (the late peaking for women, that is). I've seen a few different societies and it's the same in all of them.
My theory is that the sum of beauty and propensity for sex has to remain roughly constant for a woman if she is to maximize her chances of finding a suitable partner. Since beauty declines with age, propensity for sex has to increase.
Just my theory, though, not hard science.