We investigated whether brain activity was predictive of future reading skill and, if so, how this brain–behavior correlation informs developmental models of reading. A longitudinal study followed 26 normally developing human children ranging in age from 9 to 15 years who were initially assessed for reading skill and performed a rhyming judgment task during functional magnetic resonance imaging. Patterns of brain activation in this task predicted changes between initial and a follow-up assessment of nonword reading skill administered up to 6 years later. Brain activity in areas typically active during imaging studies of reading was found to predict future nonword reading ability, but the predictive ability of these areas depended on age. Increased activity relative to peers in neural circuits associated with phonological recoding (i.e., inferior frontal gyrus and basal ganglia) was predictive of greater gains in reading fluency in younger children, whereas increased activity relative to peers in orthographic processing circuits (i.e., fusiform gyrus) was predictive of smaller gains in fluency for older children. Interpreted within the context of a connectionist model of reading, these results suggest that younger children who are more sensitive to higher-order phonological word characteristics (e.g., coarticulations) may make greater reading proficiency gains, whereas older children who focus more on whole-word orthographic representations may make smaller proficiency gains.