My stammering started at around 5 years old. Mostly it was due to speaking very very fast. But yes it started coming suddenly. At the age of 4-5 I was very good and spoke very clearly as per my mom. Early speech therapy is the best . Make your child go to speech therapy and it will go away easily otherwise it will take more effort to remove this stammering as an adult once the speech is set in one's nature. Again I am not a therapist. I can only answer from my experience.