Tinfoil hat on? 

So they can underrepresent childhood obesity.

Nontinfoil hat? 

Metabolism changes or some shit. 

Yes, but noone really talks about it, not even the docs. Why so sudden change? 

IamSINGLE

Bell_curve

BMI takes into account weight, height and age.

The BMI scale changes significantly from age 20 to age 21.

The healthy range suddenly becomes much smaller.

HealthAndFitness

New obesity definition sidelines BMI to focus on health

BMI takes into account weight, height and age.

The BMI scale changes significantly from age 20 to age 21. 

The healthy range suddenly becomes much smaller. 

https://www.calculator.net/bmi-calculator.html