Most likely a condition called athletic heart. Difficult to detect without a test called an echocardiogram. Regular exam in a drs office will not pick it up
When I took my son into the heart center for a stress test and an echocardiogram, he leaned over and whispered to me "Mom, these are all OLD people".
Teenagers think of themselves as invincible. Only OLD people have heart conditions, etc., etc., etc.