Theories that span tasks and developmental periods require explaining how a single cognitive system can flexibly adapt across contexts yet show stable age-related improvement. Here we present a computational model that embodies a unified theory of visuospatial cognitive development. We use this model to bridge between previously disconnected domains, as diverse as infant habituation and visual working memory capacity in adults. We illustrate how the same real-time and developmental processes can account for behavior across tasks and age groups. We conclude with a discussion of the implications of a unified theory for understanding cognition and development more broadly, with an eye toward early intervention.