It sneaks up on you, really, but I have a feeling I started noticing it earlier than that. I'm 30 now and at my absolute worst. Anyway, it's just my experience. Perhaps you will remain stable or even get better. If you wanna do something about it, I absolutely recommend reading these two books: [Redefining Stuttering](http://www.mcguireprogramme.com/redefining-stuttering.php) and [Speech is a River](http://stutterers-anonymous.com/Download.html), both free and by self-described recovered stutterers, who are very well-regarded in the stuttering community. In case you're wondering why I am in such bad shape if these books are really so great, it's because you'll see that what they propose require a very dedicated commitment, which I am unfortunately not in a position to make at the moment.