I can definitely believe that - my own stutter only really developed when I was about 15. I *did* have like 3 instances of stuttering before that point, though (as in, 3 individual blocks, years apart) . I remember them very vividly since I had no idea what was going on back then. So clearly I had some sort of neurological predisposition for stuttering, and for some reason it didn't fully kick in until I was older.