The role played by the Internet in young people’s political lives has received great research attention. However, two gaps in the literature hinder the drawing of conclusions on how online political participation is related to its offline counterpart. First, although there are multiple hypotheses on the nature of the relationship, they have not been compared in any single study. Second, although the relation may differ according to developmental stage, age differences have not been examined. We address these gaps using longitudinal data from two samples of youth at different developmental stages, and test four hypotheses for each sample. It was found, among late adolescents, that online participation serves as a gateway to offline participation. However, among young adults, offline participation spills over into online participation. These findings indicate the positive potential of online political participation in youth’s political lives, and highlight the need to focus on their developmental stages.