Mine developed quite suddenly too as far as I can recall, but I was younger than you (about 15). However, I did experience some rare isolated instances of stuttering when I was a kid (like 3 speech blocks, total), so there was clearly some kind of neurological predisposition just waiting to manifest itself. My guess is that either you had this predisposition and it took a while to develop for some reason, or another issue is causing it. The most obvious and worrying culprits are things like tumors or brain lesions, but if the scans have revealed nothing so far then you're most likely fine in that area. I hope you manage to find some helpful information! Unfortunately it's quite possible that you'll never be able to get rid of it now, but you're already on the right path to a happy relationship with your stutter if you're reaching out for help, reading posts, thinking about seeing an SLP etc. If there's one piece of advice I would give you, it's to not let yourself feel ashamed or embarrassed for what is essentially a mental/neurological handicap (some would disagree on the exact wording, but you get my point). It's easier said than done, obviously, but I find it very helpful to keep in mind whenever I'm having a hard time. Ultimately most people will be receptive to what you have to say especially if you do so with enthusiasm/energy/good will/ etc., and the stutter won't seem important anymore. As you said in a reply, your confidence and experience with managing your stutter will almost certainly increase over time especially if you keep trying and maintain a positive attitude, and that should really help!