Participants ranging in age from 3 to 98 years (N = 708; approximately 60% female; 49% Caucasian, 38% Asian; 12% Other ethnicities, 1% Indigenous; modal household income > $80,000) completed a battery of tasks involving verbal ability, executive function, and perspective-taking. Wherever possible, all participants completed the same version of a task. The current study tested hindsight bias and false-belief reasoning to determine how these constructs relate to each other across the child-to-adult life span. Participants of all ages showed robust hindsight bias and false-belief reasoning errors. Hindsight bias followed a U-shaped function, wherein preschoolers and older adults showed more hindsight bias than older children and younger adults. False-belief reasoning, conversely, was relatively constant from preschool to older adulthood. Hindsight bias did not correlate with false-belief reasoning. We conclude that hindsight bias and false-belief reasoning errors are robust but unrelated cognitive biases across the life span. (PsycInfo Database Record (c) 2021 APA, all rights reserved).