We traced the developmental origins and trajectory of the hindsight bias. Three-, four-, and five-year-old children and adults identified gradually clarifying images of degraded common objects on a computer. Half the time, observers did not know in advance what the object would become. Other times, observers knew in advance the object's identity, and estimated when a na•ve same-age peer would identify the clarifying object. In two experiments, children and adults demonstrated hindsight bias by using advance knowledge to overestimate their same-age peers' ability to identify the objects. The magnitude of this bias declined across age in one experiment, but remained relatively stable over age in the other experiment. This is the first demonstration of hindsight bias in children. These findings may have important implications for children's theory of mind.