as someone approaching my 30s in america this sounds consistent with both my experience and many of my peers. our education system is more or less a trauma machine, and couple that with the demise of "third places" (places that aren't school or home for kids to hang out in without having to spend money) and the general state of the world being hard for even adult minds fo wrap around... our world is a difficult and unpleasant place to be a kid. it ain't a cakewalk being an adult either but it is relatively better with a relative increase in agency and more experience dealing with everything
The loss of 3rd places as you mentioned is huge. When I was a kid we at least had malls, where you could still hang out and roam about without having to actually buy anything. Nowadays, there's next to nowhere to just exists without spending money (unless you live outside of cities/suburbs).
Actually, the article didn't say young people were doing worse. The reason that youth is no longer one of the "happiest times" is because the study showed that people only do better and better as they age. So where before your youth would be comparatively happier to your mid-life crisis, they're saying we just get happier and happier into midlife and old age.
This pattern is driven by an increase in unhappiness among young people both in absolute terms and relative to older people.
also look at the graph in the article, the yellow line representing 2024 is signifigantly lower before 35 than any point on the blue line representing 2005-2018