In this article we address the topic of increasing partisan polarization in the American mass public, focusing on the twin influences of individual-level development and cohort replacement and the interaction between the two. We posit a model of individual development that consists of declining openness to change beyond young adulthood, an increase in party-issue constraint as age advances, and cohort-specific responsiveness to changes in the partisan environment. Results from a long-term panel study provide initial evidence of these dynamics. We then use simulations to generate expectations about how these developmental processes play out across cohorts, issues, and time. These expectations are evaluated through a cohort analysis of National Election Studies data from 1972 to 2004. Overall, our results provide a new perspective on the dynamics of individual political development and their implications for the timing, extent, and future trajectory of partisan polarization in the U.S. electorate.