I’m not sure, I think it’s different for everyone. I’m 22, and my stutter didn’t start until I was about maybe 17 or 18. However, I began “stuttering” with my handwriting at age 14, so that may have meant something in hindsight. Idk really know what to make of it. Not sure if I was born with it or not, but all of this came later for me as well. I am super grateful I never stuttered as a kid though