I was 5 or so when mine began….not unusual at all. Regarding school, a good idea to speak to staff so that they can support and guide as needed. Ask about working with children/adults who stammer. Ask about their ways of working. Their sensitivity to the nuances of stammering. For me a rule of thumb is not to focus on fluency as this sets up a child for failure, or perceived as such. Effective communication much more preferable