The influences of verbal mental age (VMA) and performance mental age (PMA) on cognitive flexibility were examined among a group of participants with Down syndrome (DS), in order to disentangle the relative contributions of each. The impaired cognitive flexibility typically observed among individuals with DS in combination with uneven VMA and PMA development suggests an opportunity to further understand the developmental relationship between VMA, PMA, and cognitive flexibility. We examined the performance of 22 participants with DS on the Flexible Item Selection Task (FIST), used for measuring cognitive flexibility among preschoolers. Partial correlations revealed that only VMA was related to the FIST after controlling for PMA, highlighting the role of verbal abilities in the development of cognitive flexibility.