Implicit learning theories suggest that we update syntactic knowledge based on prior experience (e.g., Chang et al., 2006). To determine the limits of the extent to which implicit learning can influence syntactic processing, we investigated whether structural priming effects persist up to 1 month postexposure, and whether they persist less long in healthy older (compared to younger) adults. We conducted a longitudinal experiment with three sessions: Session A, Session B (1 week after A), and Session C (4 weeks after B). For young adults, we found passive priming effects to persist and accumulate across sessions (1 week and 4 weeks). However, for older adults the effects persisted for 1 week but not 4. This suggests that for young adults, who unlike older adults experience no age-related decline in implicit memory, the limit to the duration of structural priming persistence is longer than 4 weeks. In a second longitudinal experiment with two sessions 1 week apart we found that priming in Session A affected syntactic processing in a different, independent task in Session B, both for young and older adults. Experiment 2 suggests that implicit persistence of the learned syntax is not limited to a specific context or task. Together, our findings give insight into how structural priming can contribute to language change throughout the life span, showing that implicit learning is a pervasive and robust mechanism that contributes to syntactic processing.