This is something that frightens me. What is a "diagnosable mental disorder"? I'm willing to bet this statement was far from true 50 or a 100 years ago. Not because I think people got more depressed or anxious, but rather because we're looking harder to "diagnose" things.
I think there's a big danger, especially in psychological afflictions, to always "look for" problems with yourself.
This may be a cultural thing as well. Until I came to the US (I'm from Germany), I never asked myself "Am I depressed?", "Is this what depression feels like?" when I was aving a down day. I also never considered that I may have/have had ADD now/as a kid but now people tell me I should "get that checked out" all the time.