To clarify whether the response to treatment of IgA nephropathy (IgAN) differs depending on patient age, we examined the response to treatment according to age of onset in children with IgAN. We collected data for 44 children with severe IgAN. The children were retrospectively divided into three groups based on their age at disease onset. Group 1 consisted of 24 children under 11 years old, group 2 consisted of 9 children aged 12 to 13 years, and group 3 consisted of 11 children aged over 14 years old. The clinical features and prognosis were analyzed for each group. The urinary protein excretion and serum IgA values in group 3 were higher than those in groups 1 and 2 at the most recent follow up, and histological findings showed that the MESTCG scores in group 3 were higher than those in group 1. Furthermore, the incidence of patients with persistent nephropathy or renal insufficiency in group 3 was higher than those in groups 1 and 2. Patients aged 14 years and older with IgAN may respond poorly to treatment compared with those younger than 14 years old. Therefore, care must be taken regarding response to treatment and relapse when treating older children.