I don't have the numbers, but I'm willing to wager that the number of people who stuttered into adulthood and then stopped or lessened by "growing out of it" is less than the number who stopped via a conscious psychological intervention, and possibly combined with some physical intervention as well. Take John Harrison for example (maybe that's a confirmation bias, but he didn't just grow out of it). And while it may sound cruel, it is not correct to say that an adult is completely not to blame for their current manifestation of stuttering. Word avoidance? Odd tricks like hand slapping? Blocks? None of that is natural stuttering and is learned behaviour. Now i'm not using 'blame' in the sense that they should feel bad, but if one's stutter exhibits any of the aforementioned, then of course there was some conscious decision involved down the line. And I don't think one can simply grow out of those behaviours in adulthood without conscious effort.