Child behavior problems were assessed in 1377 3-yr.-old twin pairs with the Child Behavior Check List from Achenbach (translated into Dutch language by Verhulst). The association between problem scores and maternal age at delivery of the twins was analyzed with statistical control for several potential confounding variables: birth weight of twins, maternal smoking during pregnancy, being breast or bottle fed and socioeconomic status. After controlling for all available confounding covariates, a significant linear effect for maternal age was left. Especially Externalizing behavior problems as well as the separate categories that constitute Externalizing, i.e., Aggressive, Oppositional, and Overactive, appeared to decrease continuously with increasing maternal age. This was true for both boys and girls as well as for first and secondborn twins. Evidence in support of a biological explanation of the association between maternal age and child behavior problems, is presented.