Probably in my early 20s. By 25 it was pretty manageable but of course still there. When I was 16 or 17, my mom booked me a few appointments with a hypnotist. (I truly don’t know if hypnotists are still a thing and I’m not going to try to tell you that the act of being hypnotized cured me of stuttering because that’s mainly bullshit.) the one thing that guy did for me though that I can truly say helped more than anything I’d ever tried was give me a cassette tape of a meditation guide. It was an amazing tape that ran me through breathing exercises over trance music. I can still hear it. Anyway, I listened to it every night for years and learned to control my breathing. It even got to the point where I needed it to sleep properly. Was sad when I lost it in a move. It turned out for me that my stutter was heavily influenced by breath control and anxiety. Deep steady breathing reduced speech anxiety which in turn improved my fluency. I know that isn’t everybody’s case but if you haven’t tried it, it’s worth a shot. You can find meditation tracks on most streaming service.