Recent studies suggest that the ability to use memories flexibly emerges gradually with development; however, the mechanistic changes that underlie this shift remain unknown. Participants aged 7-30 years encoded a series of related associations during functional magnetic resonance imaging (fMRI) scanning. We hypothesized that the comparatively more rigid memory behaviors characteristic of children are due in part to a failure to retrieve and link relevant knowledge to new information as it is learned. We used a pattern classification approach to show that only adults show sustained reactivation of related memories across an extended learning experience. Children show no reactivation at all, while both younger and older adolescents show an intermediate pattern, with initial reactivation that decreases as learning continues. Memory reactivation also showed different relationships to behavior across age groups, underscoring the complex nature of the development of reactivation, memory integration, and flexible decisions.