As a teen I would out eat, in physical quantity, any 2 full grown adults and not gain weight, I was 5’11" @ 125lbs by 16. I could eat several plates of whatever was in front of me, at that time my parents made food, not prepackaged processed crap. Into my 20s I’d sit down and empty a tub of ice cream, not one of them tiny ben & jerrys containers. No weight gain until I hit 28 doing a physical job and went up to 180 lbs of muscle, now I’m 150 ish and can still eat what I want when I want, tho normally I eat to live not live to eat. Calories-in-calories-out, like BMI, is only a part of the whole picture with so many unseen things affecting it, like medicines. And no, being skinny was not an easy ride.