Back to the question of why it is so difficult for an older man, let's say a 33 yo, to hook-up with an 18yo woman, say. It is MUCH harder than for, say, a 37yo man to hook-up with a 22yo woman...same age difference, each just 4 years older than in the other case.
As I mentioned above, this is known to be true statistically (in the US at least):
The mean difference in age between women and their SOs increases dramatically for women in their early 20s.
Here's my guess at an explanation: It is no news to anyone living in a modern (First World) country that people (males as well as females) pass through a stage of their lives where they are definitely not children but are also definitely not full-fledged adult members of the society. The age range corresponds roughly to the teens, and people in this teenager/"young adult"/adolescent group are not regarded as fully adult and, more importantly in this context, do not regard themselves as a part of the adult society. This is recognized in many different ways: Legal (do not have the same right as adults to vote, sign contracts, drink, smoke, view porn, hold dangerous jobs, ignore/defy their parents' wishes etc). Life activities (many still pursuing their educations, many living with their parents, many don't work or don't support a household if they do work, don't have children of their own, don't own major property, aren't sexually active or are quite inexperienced sexually, most have little experience with serious illness or injurt etc). Biological even (their brains are still developing both physically and functionally...this actually goes on at a slowing rate up until a person's made-20s). One could go on, but it is all obvious. Equally common and well known is that people in this age range typically respond by creating and living in, to the extent they can manage it, some sort of youth subculture that has its own "language" (slang which is valued precisely because "real adults" don't understand it), its own entertainment industry/performers, its own styles of dress, its own customs (at least superficial ones) etc.
When people are in this "young adult" subculture/stage, they tend to only associate closely with people of very nearly their own age. This is also well known statistically. If you ask people in their teens who their closest friends are or who they trust completely or who is important in their lives, they mostly, overwhelmingly actually, name people who are within a year or two of their own age. It is as though people's "social horizon" contracts during those years to only include those whose age is very close to their own. Anyone who is more than a year or two younger is regarded as childish and anyone who is more than a year or two older is seen as a boring and dull "old person" or "adult" who "just doesn't understand". Again, all of this is so obvious and well known to us all that it is almost silly to spell it out. But when did that ever stop me!
Finally getting to my point, most of the women is this group do not regard themselves as being fully adult and regard those who are significantly older as being in some other part of society. This raises a barrier not simply to sexual and/or romantic connections but to any close connection...even just friendship. Most people do not enjoy closely associating with those they perceive to be very different from themselves in any major way - different ethnicity, different race, different nationality, different educational level, different socio-economic level (or different "class" of society in some countries), different religion...on and on. Mostly people's social radar simply ignores those who are not "one of us"...whether we are talking about a date or a friendly poker game.
Is this a universal rule. No, of course not. There are plenty of exceptions...people who are attracted to those very different from themselves. But they are a minority, not the typical case. (It seems that I am forever pointing out to
@Solong , among others, that it is often more useful to focus on the exceptional than the typical.)
So, if you want to become close to someone in that late teen part of their lives, you either have to find one of the exceptions or you have to somehow break through the blind-spot in their social radar and get the person to perceive you as at least potentially interesting socially. If they don't, then it doesn't matter how wonderful you are (or think you are), you're simply not playing in their game...not part of their world. There are ways of doing this, but it is more challenging than getting an older person, who does see themselves as fully adult, to notice you socially.
This may seem like a very negative perspective, but in my experience, it is often useful to understand a problem if you want to solve it.
-Ww