Me too man , i also started to block stammer when i was 15 , before that i have no history of stammering. However , i do suspect that a head injury i sustain just few days before might have played a role. I went to a neurologist, he said stuttering as a only symptom is super rare as i didnt have any other symptoms that people with mild tbi usually have , so he just said it was not a cause of concern so he just dismissed me