Using a national sample of children 3 to 16 years old, this study found that the lower test scores and increased behavior problems of children born to younger mothers are not due to her age but to her family background. First, for nonfirstborn children, maternal age at first birth has a significant effect on test scores, whereas maternal age at the child's birth does not. Second, this study replicated a controversial study by Geronimus, Korenman, and Hillemeier (1994) and found that the disadvantage of children born to younger mothers is greatly reduced when maternal family background is controlled through a comparison of children born to sisters. Third, maternal age is not an important predictor of children's test score rates of improvement over time. This evidence suggests that maternal age is not causal.