Perseverance, above and beyond IQ, predicts academic outcomes in school age children, however, little is known about what factors affect persistence in early childhood. Here, we propose a formal Bayesian model of how children might learn how to calibrate effort from observing adult models and then explore this idea behaviorally across two experiments in children and infants. Results from Experiment 1 show that preschoolers persist more after watching an adult persist, but only if the adult is successful at reaching their goal. Experiment 2 and a pre-registered replication extend these findings, showing that even infants use adult models to modulate their persistence, and can generalize this inference to novel situations. These results suggest that both preschoolers and infants are sensitive to adult persistence and use it to calibrate their own effort in far-reaching ways.