The aim of this study is to analyse the impact of maternal age at first birth on low birth weight, preterm birth and low Apgar scores at one minute and at five minutes among live births delivered to primiparous Brazilian women in the city of São Paulo. Analyses were based on 73,820 birth records from the 1998 birth cohort. Logistic regression was used to assess the association between maternal age and each outcome variable, controlling for the following risk factors: delivery mode, plurality, sex, maternal education, number of prior losses, prenatal care, race, parity and community development. Maternal ages below 20 and above 30 years were significantly associated with the risks of low birth weight and preterm birth, but no association was found between maternal ages and Apgar score, with the exception that ages 15–19 reduced the odds of a low one-minute score. Even though this result seems to be inconsistent, low birth weight, preterm birth and low Apgar scores measure different dimensions of newborn well-being, and the association of each measure with maternal age is expected to diverge.