Stuttering most commonly shows up around the 3-5yo ages. But, later starts are not unheard of. The brain is developing well into your 20s, and as long as it's still developing and changing, stuff can get funky sometimes. Adolescence/puberty is a time of massive brain reorganization and it's kinda-sorta normal for stuttering to first show up at this time. It's weird, confusing, frustrating, but it doesn't mean you're crazy or hit your head. Neurologically speaking, the best thing to do is let the stutter do its thing, keep the brain as chill as possible. Think of it like a bump in the road. You feel it, but as long as you keep on driving, it won't impact your trip. Now imagine what would happen if every time you drove over a bump, you jerked the steering wheel to avoid or manage the bump. That would be far more disruptive. To the extent you can, keep speaking in your normal way and let the stutters come and go. If it's getting increasingly frustrating, worrisome, or is really impacting you, speech therapy and/or support groups can help a lot.