In my opinion it has to do with brain waves and the brain probably being at the peak capacity it can be at, this is going to be hard to explain so please bare with me. Children’s brainwaves are more active than that of a 40 year old male because Children have more energy and there brains are still taking in information. So it’s easier for children to learn how to stop stuttering while their brain waves are very active, (since stuttering mainly occurs during high brain waves or emotional events), because they’re brains are still developing. Meanwhile the 40 year old male that I used as an example now has slowed brain waves and less brain activity since it’s no longer developing, it’s reached it’s maxed size and is arguably getting smaller as it gets older. This allows for the adult to not be able to learn how to stop stuttering during high brain waves because it’s not exposed to that frequency 100% of the time. Does that make sense? Maybe it’s a more complicated way of your answer Sorry for the late reply I have notifications turned off