OBJECTIVE: We aimed to examine the ability of mother's age, and other factors measured during pregnancy (education, financial difficulties, partner status, smoking, and depression), to predict child development outcomes up to age 5 years. METHODS: Data were obtained from the Avon Longitudinal Study of Parents and Children (ALSPAC). Poor child development was defined as scoring in the worst 10% of a parent-reported ALSPAC developmental scale (ADS) at 18 months (n = 7546), the Strengths and Difficulties Questionnaire (SDQ) at 47 months (n = 8328), or teacher-reported School Entry Assessment (SEA) scores at 4 to 5 years (n = 7345). RESULTS: Only a small proportion of children with poor development had mothers aged younger than 20 years at their birth (3.3%, 6.4%, and 9.2%, for the ADS, SDQ, and SEA, respectively). A greater proportion with each measure of poor development would be identified (48.9%, 63.6%, and 74.4%, respectively) if all 6 predictors were used and a woman had at least 1 of these. Model discrimination was poor using maternal age only (area under the receiver operator characteristic curve ∼0.5 for all 3 outcomes). This improved when all 6 predictors were included in the model (ADS: 0.56; SDQ: 0.66; SEA: 0.67). Calibration also improved with the model including all 6 predictors. CONCLUSIONS: Even if programs targeted at teen-aged mothers are successful in improving child development, they will have little impact on population levels of poor child development if young maternal age is the sole or main means of identifying eligibility for the program.