Well, it's not reliable for the elderly either due to a decrease in muscle mass but I understand what you mean. Regardless of BMI's accuracy, we don't need to know someone's BMI number to realize they're fat. More than one chin? FAT. Belly hangs over your hips? FAT. People confuse you for a pregnant woman but you're not? FAT. Take up more than your fair share of the seats on public transportation? FAT. Really, the list goes on. I don't understand why fat people are obsessed over BMI as if that's the only indicator that allows us to know that they're fat. WE KNOW.