<p>Is a lack of information about eligibility rules partly responsible for the particularly low youth voter turnout in U.S. elections? In a context where new voters usually have to register several weeks before Election Day and where registration rules vary by state, this article argues that there is substantial uncertainty among young Americans about their eligibility to register and vote in elections. It uses a natural experiment that leverages the as-if random assignment of a person’s 18th birthday around a state registration deadline to identify the causal effect of uncertainty about whether someone has to be 18 by the registration deadline or by Election Day to register and vote in an election on youth voter turnout. Drawing on fine-grained data from 19 U.S. state voter files, the study finds a sharp discontinuity in turnout in nine states. There are smaller or no effects in states with same day registration, a later registration deadline closer to Election Day or explicit information that 17-year-olds are eligible to register. Moreover, the effect persists over time, with people who are discouraged from voting due to eligibility uncertainty significantly less likely to vote in future elections. These findings have important implications for our understanding of youth turnout, election reforms, habit formation, and the study of citizens’ information and beliefs about electoral rules with administrative data.</p>