I most likely am. I had many friends/relatives in my youth with bulimia and anorexia but depression and anxiety were not even in our vocabulary. I'm no expert and I'm just saying what I see, like when talking to a group of friends, except that my group of friends is HN and likes to yell at me for being real dumb.

You're not just saying what you see. You are sharing elaborate theories that are demonstrating an inaccurate and unsympathetic perspective of teenagers with mental illness.

Is this based on physician diagnosis or some objective metric like fraction of teens with BMI < X?

The fraction of teens with BMI < x is not a measure of how many teens have eating disorders. To pick the simplest possible confounding factor - teens dying of cancer would be false positives.