My doctor also checks my heart very thoroughly because my reasting heart rate is 35-37. She says my heart is like a turtle's. If you don't believe me... (I'm sorry for the big picture). My HR monitor is correct, I double check it with the finger on the wrist, too.
I'm 33 and my BMI is 18,7 but I don't believe in BMI, it's the muscle/fat ratio that matters. And of course aerobic fitness and gneteics. I have difficulties raising my HR, the hard zone for me is over 160-165.