I don’t think you can say that it’s based on evolution. I still think it’s a culture thing. You have to separate sexual desire from marriage-and-baby-birthing. It was when babies were considered part of a man’s property and as someone to pass on his acquired wealth, that a woman’s ability to birth babies belonging only to him became a selling point (and women became property to be sold).

If you look at hunter-gatherer tribes or the few matriarchal societies that have been studied, you may find less defined “this-is-what-is-attractive-to-all-of-us” ideals.

Birthing babies is only one part of this subject. Sure, teens and twenty-something’s are more likely to survive childbirth and so maybe we’ve turned that into a desire for youth. But is there truly an evolutionary reason for men to raise their biological children? By saying only biological fathers can be loving devoted fathers, you’re ignoring the caretaking abilities of adopted fathers, step fathers and loving uncles everywhere.

In Sex at Dawn, the authors suggest that people generally had no concept of how babies were made. With sperm being too small to see without microscopes and the lag time between intercourse and the initial signs of pregnancy and the likelihood that people were fucking any (hopefully willing) partner whenever the urge struck, it may have taken people a while to figure out the connection between sex and birth.

Once property ownership became a thing and people made the connection between sex and babies, and a woman’s vagina became her husband’s property so that he could ensure that her children were his to own, the desire for youth is understood. Women became investments for his future property.

But what about before that? - if you believe there’s a “before that”. What of the cultures that (until relatively recently) believed a woman’s children belonged to her? Or those cultures where the men actively raise their nieces and nephews and had no obligation to the products of their orgasms. Or the cultures that promoted the status of established mothers, not because of the potential for future babies that may belong to a potential husband, but because of her status of one with wisdom and honor because she had already added to the groups’ numbers.

Maybe it’s the push for marriage (and the cultural evolution of one man - one (or more) woman pairing) that forced culture to favor youth. I would rather see a study of a group of people who are free to have sex with whomever they want without consideration of property consolidation or baby-making or child-rearing and then get an idea for their sexual tastes. For that, you’d have to find a society that has not been touched by the big three religions, not based on property ownership, matriarchal, equalitarian, and providing equal access to food and shelter… hard to find, sure, but they’ve existed in the past.