Objective: The NIH Toolbox Cognition Battery (NTCB) is a brief computerized method for evaluating neuropsychological functions in children, adolescents, and adults. We examined how performance on the 2 executive function measures of cognitive flexibility and inhibitory control was related to performance on the other NTCB measures across development. Method: Participants were 1,020 typically developing individuals between the ages of 3 and 21 from the Pediatric Imaging, Neurocognition, and Genetics Study who were divided into 5 age groups (3–6, 7–9, 10–13, 14–17, and 18–21). Scores were adjusted for sex, level of parental education, and family income. Results: Although the correlations between the 2 executive function measures were moderate and consistent across age groups, their correlations with the other 5 cognitive measures were highest in the youngest age group and decreased across the older age groups. Exploratory factor analysis revealed that all NTCB measures loaded onto a single factor for the 3- to 6-year-olds. Across the older age groups, the executive function and processing speed measures loaded onto one factor, and the vocabulary knowledge, oral reading, and working memory measures loaded onto a second factor. Conclusions: These results indicate that younger children’s performance on the NTCB is more intercorrelated and less differentiated, while performance on the NTCB executive function measures becomes more differentiated from performance on the other measures with development. These results support the hypothesis that executive functions become increasingly differentiated from other cognitive functions with development as the functional specialization of neural systems progresses throughout childhood and young adulthood.