A few things:

• experience. There’s a stereotype that older women are more experienced when it comes to sex and know their way around better than a younger woman would in pleasing a man. I think why that doesn’t go the other way is that men stereotypically get more lazy as they get older, finding ways to do less to get what they want or other ways (financial; sugar daddy) and they become less attractive whereas older women can be just as attractive, if not more, as they age or at least know how to maintain their beauty as they age.

• a bit of taboo. A lot of guys with “mom’s friends”/teacher fantasies. It’s hot to think about one of your mom’s hot friends hitting on you to some guys. Then there’s the taboo of engaging in sexual activities with someone much older than you like your teacher in a setting that isn’t socially acceptable. We like what we can’t or shouldn’t have, often times. Speaking as a guy and having male friends, we’ve talked a bit about our fantasies like this and most of us have these fantasies.

• relative age. You may be younger in your 20’s but most men consuming porn are probably much older and the women who are older to you are actually their age. So it’s not always “milfs” because what is a milf to you is just a woman to a guy of an older age and he may not be interested in younger women and just into women around his age.