I imagine a lot of it, in males of that age group, could be called “A loss of confidence of purpose”, and in females, “A loss of social support”.
I distinguish between the two generalizations, both because they are recognized societal expectations, and because both have known biological consequences.
To explain, normal males have a biological prerogative to propagate their DNA; females to both get the best male DNA for their offspring and to get a male provider to help them raise their offspring. This is the underlying biological purpose of marriage, that gives a better outcome to males, females and their offspring than just basic reproduction.
To accomplish this, males have to focus on successful provision, both in competition with other males, and in cooperation with them in an organization or system. This means that males become emotionally dependent on this system, and suffer accordingly if it breaks down. “I’ve been fired, so I’ll kill myself”, is the attitude.
Females, however, are equally dependent on the female social structure, a decentralized support network back at “the base camp”, and on the males who “are out hunting.” If she loses support of this network, and/or her husband-provider, she and her offspring can be in deep trouble. “I or we have lost support, so I’ll kill myself, and maybe the kids, too.”
Variations on this happen even after child rearing is no longer an issue, especially now, when the original paradigm was so bitterly attacked during their childhood. In other words, both the male and female paradigm were strongly attacked in the 1960s.
Sex was for pleasure, and only incidentally for procreation. Females would provide just as well as males, so much that they really didn’t need males. Females no longer needed to network and provide each other mutual support. Marriage could be dispensed with, because it was merely a religious convention, and had no real purpose. Males were purely in competition with each other and didn’t need a cooperative system of employment or loyalty to each other.
In other words, trying to dispense with millions of years of biology, because it was assumed that it was just cultural baggage.
So it is no wonder that children raised in such chaos now find themselves, as adults, without individual or social purpose, alone in a crowd of equally detached people, bereft of sexual relationship as well.
Add to this the obvious and well known dilemmas, such as the emptiness of materialism, the decrepit spiritualism of many religions, and the nagging discomfort of imbalance and disorder of society.
I’m surprised that the suicide rate among such people is as low as it is.
I think the dissonance formed between being part of a demographic which is positive (productive, not criminal, innovative, etc.) and yet being the one acceptable target for humor because of what you are may be a significant driver of this phenomenon. This may not amount to a fully developed theory - but perhaps some epidemiologist could get on it.