This paper analyses the effects of maternal age at birth on children's short and long-term outcomes using Finnish register data. We exploit a school starting age rule for identification. Mothers who are born after the school entry cut-off give birth at higher age, but total fertility and earnings are unaffected. Being born after the cut-off reduces gestation and, hence, child birth weight. The effects on birth weight and gestation are rather small, however, suggesting that the long-run impacts may be limited. Accordingly, we find no impacts on longer-term child outcomes, such as educational attainment and adolescent crime rates. Thus, using this source of variation, we find no favorable average effects of maternal age at birth on child outcomes.