Much recent work has shown that children actively shape their learning progress by choosing what, when and from whom they want to learn. However less is known about whether children are sensitive to gaps in their knowledge, and whether they selectively solicit information about items they previously indicated having a knowledge gap about. In a cross-situational word-learning task, we asked 5-year-olds, 6-9-year-olds and adults to estimate their knowledge of newly learned word-object associations. We then examined whether participants preferentially chose to hear the labels of objects they reported not knowing the label of and whether such uncertainty-driven sampling behavior led to improved learning. We found that participants in all age groups were sensitive to gaps in their knowledge of the word-object associations, i.e., were more likely to say they had correctly indicated the label of an object when they were correct, relative to when they were incorrect. Furthermore, 6-9-year-olds and adults were more likely to sample objects whose labels they reported not knowing, while 5-year-olds were equally likely to sample objects whose labels they reported not knowing and knowing. In other words, older children and adults displayed sampling behavior directed at reducing knowledge gaps and uncertainty, while younger children did not. However, while all age groups showed evidence of having learned the word-object associations, participants who displayed more uncertainty-driven sampling behavior were not more accurate at test. Our findings underscore the role of uncertainty in driving 6-9-year-olds’ and adults’ sampling behavior and speak to the mechanisms underlying previously reported performance boosts in active learning.