I can tell you from experience you don’t grow out of it and at least for me there is an ebb and flow with severity. Mine bad years seem to be induced by extreme long term stress. For instance after high school I had a couple years where I was very fluent and had almost no issues with my speech. But when I hit my last year of college with end of degree projects and the prospect of looking for jobs it grew worse and worse. Now it’s been 3 years since I’ve graduated I was lucky enough to get a great job right out of college and I’m just now starting to become more fluent again.