We investigated phonemic competence in production in three age groups of children (7 and 8, 10 and 11, 12 and 13 years) using rhyme and phoneme monitoring. Participants were required to name target pictures silently while monitoring covert speech for the presence or absence of a rhyme or phoneme match. Performance in the verbal tasks was compared to a nonverbal control task in which participants monitored tone sequence pairs for a pattern match. Repeated-measures analysis of variance revealed significant differences between the three age groups in phoneme monitoring, whereas similar differences were limited to the younger age groups in rhyme monitoring. This finding supported early and ongoing acquisition of rhyme- and later acquisition of segment-level units. In addition, the 7- and 8-year-olds were significantly slower in monitoring phonemes within consonant clusters compared to the 10- and 11-year-olds and in monitoring both singleton phonemes and phonemes within clusters compared to the 12- and 13-year-olds. Regression analysis revealed that age accounted for approximately 30% variance in the nonverbal and 60% variance in the verbal monitoring tasks. We attribute the observed differences to the emergence of cognitive processes such as segmentation skills that are critical to performing the verbal monitoring tasks.