Kids tend to start stuttering when they begin to use sentences. What happens is the brain has to manage sentence formulation and longer sequences of sounds. So saying “go park” at two years old is fluent. Saying “I wanna go park” is more complex and later “I wanna go to the park.” More complex still. The language and articulation demands pile up and that’s usually when dysfluency starts. Around 3-5 years old. Plenty of kids will simply outgrow the dysfluency in a few months or a year. Around 4 or 5 is also when kids become more self aware and self conscious. If the child starts to get a lot of embarrassment and anxiety about their speech full blown stuttering occurs.