I developed a stutter when I was 13, so not mid or late teens. To be completely honest, I don't have a clear cause for it, but it sucked at the time. Now it's just normal to me. I think maybe seeing a speech therapist would be a good idea, but many speech therapists just focus on fixing a issue, not finding the cause. I think visiting a psychological therapist would be a good idea, more so than a speech therapist. My only emotional advice is to stay strong, and try to accept it for the time being. You aren't any less intelligent for developing a stutter, since I felt that way about myself at the time. And surround yourself with people who accept it, or don't mind. I made the mistake of keeping many friends who belittled me for stuttering. Sorry if this is scatter-brained, I don't usually make posts like this lol.