As adults, we do not expect ignorant agents to behave randomly or always get things wrong. Instead, we expect them to act reasonably, guided by past experiences. We test whether 4-to-6-year-olds share this intuition and use it to infer others' knowledge, or whether they rely on a simple "ignorance = error" heuristic identified in past work. Across three pre-registered experiments (n = 264 4-to-6-year-olds recruited in the US between 2018-2022; demographic data not collected), we find that 4-year-olds expect agents to draw on past experiences when acting in new situations. However, only 6-year-olds reliably use this expectation to infer others' knowledge from behavior. These findings suggest that by age 6, children use a causal model of how ignorance shapes behavior, and not just a cue-based understanding of epistemic states.