Data from the national standardization of the French version of the WISC-III were analyzed to determine when during childhood the IQ-related process of differentiation appears and how the strength of the relationships among subtests evolves with age in low- and high-IQ groups. Indeed, some recent studies suggest that age might moderate the effect of IQ on the differentiation of cognitive abilities. The whole sample (N = 1, 120) was divided into six groups based on age (7-9, 10-12 and 13-15 yr) and on the mean of the subject's standard scores for Vocabulary and Block Design (mean less than 10 vs. mean greater than or equal to 10). Correlations between the remaining eleven subtests were then calculated for each group. Results showed an age-related increase in relationships among subtests for low-IQ participants, but good stability for high-IQ participants. This confirms the hypothesis of an Age x IQ interaction on the degree of differentiation of abilities and suggests that persons with low IQ do not exhibit a less differentiated pattern of intellectual abilities from the outset of their development. Rather, this characteristic seems to appear in late childhood.