I've been lurking on Reddit for over a year now but thought that this discussion warranted my registering here. Just like OP, my stuttering appeared in my teens. I have no recollection of it actually developing, just that in one month I didn't have it, and then in the next month I did. I've had it for around 12-13 years now and although with work colleagues or friends it is mostly masked, the stammering takes a life of its own when I'm around my family. I too don't know why the sudden change. Same thing happened to my mother about 15 years ago. No stuttering for 30 years and then BAM!, out of nowhere. My grandparents had diabetes, strokes, what not. But not my parents thankfully!