Understanding how the structure of cognitive abilities changes depending on age and ability (age differentiation and ability differentiation) has critical implications for cognitive ability assessments and cognitive developmental theories. Most differentiation research has focused on general intelligence; however, as children increasingly invest in specific domains and school-taught subjects, we argue these investments should rather affect their domain-specific ability structures. This study capitalized on a representative longitudinal sample of 17,979 children from the U.S. who were assessed in mathematics, reading, science, working memory, and cognitive flexibility at seven waves (from Kindergarten entry to fifth grade). We applied longitudinal non-linear confirmatory factor analysis with Montecarlo integration to analyze the data. The non-linear model was set up as a bifactor model in that all tests were regressed on the g factor, but each test score also loaded on one of five orthogonal content-specific ability factors. The results revealed that loadings on a general intelligence factor remained similar but domain-specific factor loadings increased on most tasks during the kindergarten–fourth-grade period before dropping in fifth grade. Hence, age and ability differentiation are conceptually distinct, with the former pertaining to specific abilities and the latter to general intelligence. Further, children compensated for lower general intelligence with higher levels of domain-specific abilities. Our findings were robust to the addition of autoregressive paths, which likely contribute to the stability among achievement tests in the early school years. Overall, our study motivates a more nuanced understanding of children’s cognitive development.

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