In 2000 Census microdata, various outcomes of second-generation immigrants are related to their parents' age at arrival in the United States, and in particular whether that age fell within the "critical period" of language acquisition. We interpret this as an effect of the parents' English-language skills and construct an instrumental variable for parental English proficiency. Estimates of the effect of parents' English-speaking proficiency using two-stage least squares yield significant, positive results for children's English-speaking proficiency and preschool attendance, and significant, negative results for dropping out of high school and being below age-appropriate grade. (Contains 4 figures, 6 tables and 41 footnotes.)