Interesting. For me I didn’t start stuttering until about 21, and am 25 now. However I think your theory still kinda holds, in that I was afraid of people’s reactions to my stuttering, which I think exasperated it. Though at this age, we have more of a conscious, so rather than see anxious faces, I just assumed that people will react differently to anything different than typical behavior. I also believe I have the temperaments and some biological factors as well.