wtf?
ive never had serious injuries in my life. though my childhood was pretty active
its just bit other shape of knee, it is moved towards middle a cm or two, very common.
you don't have to do extreme sports while it happened, just turned badly, and once it happened it will happen again most likely, cuz its then stretched, most likely surgery is needed.
that is just bad luck, i cant call my childhood very active, but ive had a broken + dislocated ankle in 2 places, and this.