I got stutter since kindergarten because I once tried to mock someone who had stutter. I don’t have any parents or close relatives who also have this problem. Also, I know a lots of my friends who got theirs when they got scared from a horror movie or even thunderstorm! One of my best friends got his while singing with an artist who was intentionally stuttering in his song :)) So, that means it’s either by imitation or a sudden shock or stress at the very young age. Though, it might also happen for adults who didn’t have any of these experiences but they had a unexpected sudden event that shocked them so much and they started stuttering and it sticked to them.