Stammering can change as you grow up, and this might have to do with a few things like how children learn language, their hormones, and the fact their brain is developing. It may also be connected to changes in what they are expected to do at school and their social life, which is all part of growing up.
As children get older, they keep getting better at using language. At school age, they start using more academic words, sharing their thoughts in class, and making new friends. When they become teenagers, the words they use in school become even trickier, and they're expected to do more public speaking and argue convincingly in class. Plus, they might feel more pressure from their friends and become more aware of their speech, which can make speaking in social situations feel a bit worrying.
Sometimes, when young people are going through all these changes and learning new things, their stammer might become more noticeable because there's more pressure to speak clearly about complicated stuff.
We're still learning more about stammering and how it works, but these are some of the reasons why a child's stammer might change as they grow up.