This thread is full of people talking about cultural, societal, and economic factors that may influence this outcome. I've yet to see anyone mention the factor that I wish we more thoroughly investigated: health.
This has been discussed in some articles at length, but we still haven't identified primary causes, but testosterone levels are falling in men (at a given age) at a pretty steady pace year over year. This is likely associated to the prevalence of endocrine disrupting chemicals in our food and water supply, such as BPA in plastics and PCBs in common food-oriented coatings (Teflon). Adding on to that, there are also phytoestrogens from soy lecithin in many food products, and we also had periods where growth hormones were commonplace in dairy products. There is a growing body of evidence, not yet fully interconnected causally, that seems to indicate that the introduction of these sorts of hormones and endocrine disruptors into our food and water supply has had marked effects on health, everything from the increasing epidemic of obesity and obesity-related illnesses like diabetes, to earlier ages of menarche in young girls, and increasing prevalence of gynecomastia in young boys.
I, for various reasons, know numerous people who fall soundly into the NEET stereotype here in the US, still living at home (or in extreme poverty) playing video games and doing menial labor well into their 40s, despite having marketable and useful skillsets that would provide them gainful employment. Without exception, every person I know like this suffers from mental health issues, predominantly anxiety, all are obese, and all consume and have consumed throughout their life heavy amounts of processed food and take-out fast-food, which both increase exposure to chemicals like Teflon (often used to line take-out containers). Their generally unhealthy diet and lifestyle throughout childhood and into adulthood is likely a key factor in the outcome of their life, clearly affects their general health as adults, and is likely a contributor to their mental health issues and defeatist attitudes.
The absence of any serious public health movement to improve the food and water supply of all Americans makes it unsurprising to me that we now find ourselves in the position we are in. Compounding this with the absence of universal mental healthcare and the comorbidity between depression, anxiety, and obesity, makes it unsurprising as well.