This chart shows the result of a survey of 194 people with misophonia who were contacted through the online misophonia groups and websites. This shows that the most common age for onset of misophonia is in the pre-teen (late childhood) years and it usually becomes severe a few years later.
Note that misophonia can begin at any age. This basic finding has been reproduced by several reserach studies. So even though misophonia develops most often between the ages of 8 and 13, it is not something that simply occurs with the passage of time.