I think there are two factors. 1, men and women are about 10-20 years apart in terms of when they peak in eligibility. 2, they're also about 10-20 years apart when it comes to aging.
A man at 18 has potential, but is not super attractive as a long-term romantic partner. He needs to work hard to achieve this. A woman at 18 is almost peak attractiveness as a long-term partner. She needs to work hard to maintain this.
Men are probably peaking at 30-40, in terms of attractiveness as a long-term partner, while women have already lost a lot of attractiveness by this age.
This is why it seems so natural that men should find younger women attractive, while women find older men attractive.
In terms of aging, while a woman will begin to look very old by her 50s, a man will still look OK. But once a man reaches 70, he's probably going to be showing his age also.
I think that one of the problems with relationships in the modern West, is that while there was this kind of imbalance, there was a kind of deal where if the women was chaste, the guy stayed with her forever, even when she was old and unattractive while he was still fairly eligible. Now that women want to sleep around until their 30s, men are losing interest.