Using longitudinal Young Lives data from Andhra Pradesh and Telangana, India, we examine the effect of noncognitive skills on cognitive abilities at the onset of adolescence and later adolescence, and address endogeneity by using siblings’ noncognitive traits and primary caregiver well-being as imperfectly excluded instruments for the focal child’s noncognitive abilities. Our results suggest that agency improves mathematical ability at the onset of adolescence, indicating a sensitive period of development. No similar effect on language abilities at the same stage indicates domain-specificity of agency effects. Self-efficacy has no effect on cognitive outcomes at either stage. We are unable to reject the null that agency and self-efficacy have no effect on cognitive abilities in later adolescence. Agency effects are concentrated among boys, consistent with theories of gender-specific socialization.