This is the first population-based study to examine the effect of maternal age on early child health in Asian countries, where the rise in average maternal age at birth has become a serious concern . We found decreased risks of unintentional injuries and hospital admission at 18 months’ follow-up according to older maternal age in both the 2001 and 2010 cohorts. These trends seemed to be robust regarding the adjustment of detailed socioeconomic circumstances and potentially mediating factors such as preterm births and birthweight. We did not observe marked differences in hospital admission at 66 months once we controlled for biological and socioeconomic factors in the 2001 cohort. A key strength of our study is the use of a nationally representative sample of Japanese children from two large birth cohorts. Results were consistent between the two birth cohorts despite the 9-year gap, strengthening the validity of our findings.