I made this argument not too long ago:

So why do human males prefer younger women? Evolutionary psychology always has this dumb answer: Youth equals fertility. But if this is a natural law for us civilized humans, why is it not the law of the jungle? It seems we have completely mistaken a cultural phenomenon for a biological one. It is very likely that the desire for young women has no natural basis but is instead entirely a cultural construction. Indeed, the best sociobiologist in the business, Sarah Hrdy, goes as far as to see this strange kind of desire as emerging from the cultural institution of marriage. (In her brilliant paper "Female Sexuality and the Prehominid Origins of Patriarchy," Hrdy argues that youth has its value in the context of a long-term investment.)

Though I still agree with Hrdy's thinking, my position has been readjusted by something I came across while reading a new book by the evolutionary biologist Mark Pagel, Wired for Culture: Origins of the Human Social Mind. Pagel's book is not about sex or marriage but the whole picture of human sociality. But in one part of the book, he mentions "the grandmother hypothesis" in connection with the "rule of two," a population law that humans have escaped ultimately by cultural means.

I do not want to get into Pagel's ideas in this post (I will review his book in the near future); I only want to point out something I had to take seriously when I connected the solution of the grandmother hypothesis to this question: Why do males in all human societies tend to prefer younger females, when males in chimpanzee, bonobo, and gorilla societies do not share this preference. For Hrdy, the fact of this curious distinction pointed in the direction of cultural sources rather than biological ones. I agreed with her. But I've had to adjust this agreement with some thinking about menopause and its social implications.

Comes down to this: It's almost impossible for a human female to safely give birth without assistance (one of the roots of human sociality). Why? Because human babies and their heads are too huge. As Jared Diamond points out in his short book Why Is Sex Fun:

A one-hundred-pound woman typically gives birth to a six-pound infant, while a female gorilla twice that size (two hundred pounds) gives birth to an infant only half as large (three pounds). As a result, human mothers often died in childbirth before the advent of modern medical care, and women are still attended at birth by helpers (obstetricians and nurses in modern first-world societies, midwives or older women in traditional societies), whereas female gorillas give birth unattended and have never been recorded as dying in childbirth..

Also, the older you are, the more difficult and deadly is childbirth. This is not the case with other apes. Older female gorillas, for example, can give birth with far less danger to themselves than can older female humans. With humans, youth means a greater chance of surviving the process of birthing a being with an abnormally big head. This means the desire for young women might also be a fear of death.

I want to say more, but I will leave it at that for now.