One. They lived through one major recession, and here it wasn't really major, and is not something they lived through recently. They were younger than 6 when it started. I've lived through two. The 80s weren't a happy time, and there are no signs that that had such an impact on teen mental health. If it did, it would be proof that the impact of a major recession is temporary.
> prospect of a mountain of debt right after graduation
Situation wrt to teen mental health is similar in Europe, but there's a much smaller graduation debt.
Don't try to downplay the role of internet, mobile devices and social media. Here's a thought for your correlationism:
> Teens are aware that they are going to inherit a pretty terrible world, and are powerless to do anything about it.
Perhaps it's social media that makes that they are either too aware of it or simply mention it more often.