It is much rarer but possible for someone to start stuttering as an adult. This can happen after a brain injury, drug use (some psychopharmaceutical drugs have this side effect), sleep deprivation, intense stress… As opposed to developmental stuttering which starts in childhood, aquired stuttering is treatable. One of my patients started stuttering in his 20s, most likely because he was on medication for his depression. His treatment was completely successful and 15 years later he is still stutter free. Another patient started stuttering after a brain trauma and stopped stuttering half a year later after intensive rehabilitation.