I will tell you this. I'm 23, been stuttering since probably 7 years old. It has diminished a lot, massively, over time. What it was, for me? Self-confidence. Real self-confidence, because I'll tell one thing, it's something that gets built inside of you, very very very slowly. For me, those quick methods (like do something outrageous in public), do not work. They work only temporarily, but they are trash overall, I've tried them. The way I understand it for now, is that...if you care about the judgement or reaction of the people in front of you, you will stutter. If you don't care, you just don't stutter. For me stutter is like performance anxiety. The more confidence you have, the less you care. The best first step, to increase confidence, imo, is: martial arts (or in alternative, other types of physical exercise), for most people they can't even imagine how strong they can be. When you see how strong, powerful and capable to defend yourself you can be, you realize that there's no need to be anxious with people (and you will also feel the urge to defend the weaker). For real, it's a game changer.