Is it just the young men whose mental health is declining? I'd go a generation earlier and see how things start to get pretty bad. A lot of kids these days don't hang out in person, they are hanging out virtually while laying down in their beds or in comfortable chairs in front of their consoles or computers playing video games online. This is quite unealthy way to grow up. A lot of teenagers, male or female are also undetachable from their cellphones and social media too. This is also a very unhealthy way to grow up.

Even when assuming that a decline in hanging out in person is a huge factor, it is particularly bad for the youngest. Because all the suburbia+birthrate trends get much worse at the point where the number of same-age peers within comfortable reach of available transportation drops from 1 to 0. The same basic principle was less awful for the earlier generation that more likely lived through a comparatively mild 3 to 2 drop, and remains less bad for those that grew up just in time to make up for the distance with reasonable affordable petrol.

Not comfortable chairs! Back in my day, we had to walk up hill to school both ways, in the snow!