Stereotypes about science, technology, engineering and mathematics (STEM) are associated with reduced STEM engagement among girls and women. The present study examined these stereotypes from early childhood through adolescence within informal science learning sites (ISLS; science museums, zoos, aquariums). Further, the study explored whether interactions with male or female educators influenced STEM stereotypes. Participants (n = 997, female = 572) were ISLS visitors in the UK and USA who either interacted with an educator, or no educator. With age participants were more likely to report that “both boys and girls” are “usually”, “should” be, and “can” be good at STEM. Independent of age, male participants reported that their own gender group “should” be good at STEM. Educator interactions did not influence stereotype responses. These results highlight early childhood as a key developmental window in which to challenge ideas about who can and should be proficient in STEM.