False-belief reasoning, defined as the ability to reason about another person's beliefs and appreciate that beliefs can differ from reality, is an important aspect of perspective taking. We tested 266 individuals, at various ages ranging from 3 to 92 years, on a continuous measure of false-belief reasoning (the Sandbox task). All age groups had difficulty suppressing their own knowledge when estimating what a naïve person knew. After controlling for task-specific memory, our results showed similar false-belief reasoning abilities across the preschool years and from older childhood to younger adulthood, followed by a small reduction in this ability from younger to older adulthood. These results highlight the relative similarity in false-belief reasoning abilities at different developmental periods across the lifespan.