Background: Command of language is a fundamental skill, a cornerstone of multiple cognitive and socioemotional aspects of development, and a necessary ingredient of successful adjustment and functioning in society. Little is known about the developmental stability of language in at-risk youth or which biological and social risk factors moderate stability. Methods: This four-wave 10-year prospective longitudinal study evaluated stability of core language skill in 1,780 children in varying categories of biological and social risk in a multiage, multidomain, multimeasure, and multireporter framework. Results: Structural equation modeling supported loadings of diverse age-appropriate measures of child language on single latent variables of core language skill at 15 and 25 months and 5 and 11 years, respectively. Core language skill was stable over the first decade of life; significant and comparable stability coefficients were obtained for children with diverse biological and social risks, including poor health, welfare status, teen motherhood, ethnicity, gender, birth order, and families that changed in income and maternal education over the study period; stability in language was strong even accounting for child nonverbal intelligence and social competence, maternal education and language, and the family home environment. Conclusions: Core language skill varies in stability with age but is robustly stable in children regardless of multiple biological and social risk factors.