My uncle completely lost his severe stutter in about a week aged 19. it literally just vanished. I also have a friend at church who worked on his stutter with a breathing technique in his late 20s and then basically stopped stuttering by his mid 30s. He’s about 60 and I’ve never heard him stutter (in all 4 languages he speaks!). I personally have had periods of almost complete fluency followed by periods of debilitating severe stuttering. Because of these periods of fluency I have faith that at some point my mentality will shift and I will just be fluent again. But my goodness is it a nightmare trying to endure the stuttering in the meantime!