The best theory I’ve come across to explain everyone’s varying experiences with stuttering is everyone has the capacity to stutter. Some people have a higher capacity than others . Also everyone has a trigger for their stutter. So if you truly didn’t stutter before 11, that probably means you were triggered at 11 and haven’t been able to break the habit since. This is just the best explanation I’ve come across because the real answer is no one still knows why we stutter.