In the case of my teenager,, I reckon it's the same depression/anxiety that has run in the family for at least 4 generations, though she is the first that will feature in any statistics because she is the first to have got help for it. I hope her life will be all the better for it.
Dd didn't have access to social media until long after she had clearly developed issues.
As a matter of fact, I do remember several of my peers (both in the UK and my country of birth) developing MH issues in the 70s when I was a teen. Plenty of anorexia around, plenty of depression.
My MIL was medicated for depression as a young woman in the 40s/50s.
Earlier than that, there were serious concerns about young women's MH in the late 19th/early 20th century; it's quite a theme in literature of the period. Remember Sigmund Freud- he didn't get famous because the young people he saw were so mentally resilient.
Not denying that there is a lot of pressure on teens these days, and that many aspects of our lifestyle can hardly be described as healthy, but I do think it is wrong to assume that all MH issues must be caused by whatever is the fashionable explanation at the time (in the Victorian age, it was assumed that it was the lack of anything useful to do).