The transaction of children's core language skill and their home learning environment was assessed across 5 waves from infancy (15 months) up to adolescence (11 years) in 1,751 low-socioeconomic status families. Child core language skill and the quality of the home learning environment were each stable across waves, and the two covaried at each wave. Over and above these stabilities and concurrent correlations, and net child social competence and maternal education, higher quality stimulation and support in the home learning environment at each wave advanced children's core language skill at each subsequent wave, and reciprocally children with more advanced core language skill at each wave stimulated a higher quality home learning environment at each subsequent wave. These transactions were robust across child gender, ethnicity, birth order, and developmental risk. This bidirectionality shows that children consistently affect their environments from infancy to adolescence and underscores that the home learning environment is a worthy intervention target for improving core language skill in children regardless of age.