Abstract: Humans flexibly adjust their reliance on model-free (habitual) and model-based (goal-directed) strategies according to cost‒benefit trade-offs, the ability of which is known as metacontrol. Recent studies have suggested that older adults show reduced flexibility in metacontrol. However, whether the metacontrol deficit in aging is due to cognitive or motivational factors remains ambiguous. The present study investigated this issue using pupillometry recording and a sequential decision-making task with varied task structures and reward stakes. Our results revealed that older adults performed less model-based control and less flexibility when the reward stake level changed, consistent with previous studies. However, pupillometry analysis indicated that older adults showed comparable sensitivity to the reward stake. Older adults varied in task structure knowledge according to their oral reports, and the subgroup with good structural knowledge exerted a similar pattern to younger adults. Computational simulation verified that poor structure knowledge representation impaired metacontrol. These results suggest that the inflexible metacontrol in the elderly population might not be due to motivational factors but rather poor structure knowledge.