My college age son—a very tall and thin distance runner—has a BMI of 15. He had that same BMI when younger as well; he’s been tall and thin all his life, even though he always eats well. An idiot PA in college “reported” him to his team and accused him of having an eating disorder and required him to get a bone density test (of course his bones were fine), and his pediatrician had to write a note to his coach verifying his BMI was normal for him, and had always been 15 (or even lower).
BMI is not one size fits all. Some people are just thin—and especially so if a distance runner.
When I was younger I was 5’9” and weighed 113. Not an ounce of fat on me and couldn’t gain a pound if I tried. Everyone thought I had an eating disorder. I did. I ate everything in sight. lol I’m still slim but have to kind of work on it now. I survived breast cancer but my chemo cocktail had something in it I was allergic to so they pumped me full of steroids to counteract it. Blew me up big time. Kind of ruined my metabolism I guess. That and age.