My theory is as following:

-Boys aged 13 up until men aged 93 are generally most attracted to the same type of women: Healthy women in their early twenties. These are women who are most likely to bear them children. Men who are most attracted to women in their forties probably wouldn't have as many children as men attracted to women in their twenties.

-Girls, from age 13 up until women aged 93, are most attracted to men who are slightly older than them.

-It follows from this, that men of all ages are "competing" for roughly the same pool of women: Women aged in their early twenties. There may be plenty of men aged in their fifties who will marry a woman aged in her forties, but this is largely due to the fact that they see women in their twenties as unavailable to them.

-It follows from this, that as the life expectancy of men and women increases, a growing number of women find themselves living up until ages where they become unattractive to men. Similarly, the number of men who compete for women in their twenties, increases drastically.

-As the population ceases to grow, the number of women aged 20 for every man aged 40 begins to decrease. In a population that's still growing, younger cohorts are larger than older cohorts. When the population stabilizes, or worse, shrinks, this ratio is reversed.

-As society becomes less violent, young men are less likely to die. In earlier eras, high death rates for young men in dangerous jobs and violent conflicts ensured that competition for young women was relatively limited.

-The abnormal ratio between men and women of different ages has harmful psychological consequences for men and women of all ages, as well as harmful social consequences. Young women in their twenties enter a relationship market that's heavily stacked in their favor. As a result, when they enter relationships with older married men, they're capable of placing high demands on these men: Before there's any sex, the man first has to leave his wife. If the man refuses, she can quite easily find another older married man looking for a young woman. As a result, older women married to successful men find themselves very anxious that they might be abandoned by them. In fact, many of them may initiate a divorce beforehand as a result. Women of all ages become excessively anxious about the prospect of growing old.

-Young women in their twenties, as long as they are not very unattractive, tend to find that they have an abundance of men available to choose from, from all ages and walks of life. As a result, they gain very high expectations of what a man has to live up to. When a young woman no longer perceives entering a long-term stable relationship as a tough challenge, all that remains is to see such a relationship as a ball and chain around your ankle. Note that the common Victorian trope of feminine anguish about the prospect of lifelong spinsterhood seems to have nearly vanished from our society. Although a young woman may be aware that her high demands might not be viable any longer as she grows older, it becomes psychologically difficult to settle for less. Many of these women thus decide to settle for a Phd, a career or a large number of cats, as they enter their thirties.

-Older men still find themselves attracted to young women, but for most of them, these young women are ultimately unobtainable. This generates harmful psychological consequences in these men: Guilt towards their wives, guilt towards their children, a feeling of being sexually perverted, a feeling of being inadequate, deep dissatisfaction with their situation in life. This probably plays a role in rising suicide rates among these men.

-Older men begin to monopolize wealth and resources, to increase their own chances in the relationship market at the cost of younger men. As a result, wealth disparities increase and young men's chances on the labor market decrease, while young women's chances on the labor market increase. The purpose this has for older men is to make these young women, who might be working as "human resources specialist" or "personal assistant", more sexually available to these older men who have successfully monopolized wealth, employment opportunities and other resources.

-Most young men begin to perceive themselves as having relatively low chances on the relationship market. The exception to this are young men who have an inflated self-image, who don't recognize their own poor odds. As a result, men with an inflated self-image become more likely to enter relationships than men with a realistic self-image. As a result, all sorts of socially undesirable men (narcissists, psychopaths, etcetera) procreate at increased rates. In societies like Japan, where men are more modest and contemplative, large numbers of young men simply give up on relationships altogether. We thus find a large number of young men now mysteriously missing from the labor market. Most of them have succumbed to neckbeard syndrome, invisible to authorities as they play video games in their parents basement most of them time. Some young men succumb to steroid abuse, as they wish to live up to unrealistic standards.

-Societies begins to suffer various mysterious ailments. Economic growth ceases, fertility rates continue to decline, suicide rates and drug abuse increase, divorce rates rise, marriages rates decline. Society is in decline and people feel unhappy, but they can't quite figure out what's going wrong.

-All of the above trends receive an extra boost, when developed countries decide to import large number of men from third world countries, as cheap labor or as refugees, who end up competing for the same limited pool of women. All of these trends soon will begin to reveal themselves in China and India too, where sex-selective abortion has made the problem even worse.

Addendum:

For a healthy demographic pyramid, check out Scotland in 1861 here. Note how there are far more women in their twenties than men in their twenties, as well as relatively few middle-aged men compared to young men.

For an unhealthy demographic pyramid, check out Sweden in 2014. Note how there are more young men than young women. Note how there are about as many men in their forties as there are men in their twenties.

For a catastrophic demographic pyramid, check out Japan here. Note how there are more men than women at every age, until we reach retirement. Note the abundance of men in their forties, preying on young women in their twenties. A recipe for neckbeard syndrome in young men, middle-aged male suicides and deeply unhappy anxious women.