Many social interactions require the synchronization--be it automatically or intentionally--of one's own behavior with that of others. Using a dyadic drumming paradigm, the authors delineate lifespan differences in interpersonal action synchronization (IAS). Younger children, older children, younger adults, and older adults in same- and mixed-age dyads were instructed to drum in synchrony with their interaction partner at a constant, self-chosen tempo. Adult-only dyads showed the highest and children-only the lowest levels of IAS accuracy. It is important to note that children improved reliably in IAS accuracy when paired with older partners. The observed age-related differences in IAS accuracy remained reliable after statistically controlling for individual differences in the ability to synchronize to a metronome and for between-dyad differences in tempo. The authors conclude that IAS improves from middle childhood to adulthood and that adult interaction partners may facilitate its development.