Survey-based research consistently links outsider candidates and political mavericks to disproportionate youth support, yet data constraints have prevented scholars from translating this into clear behavioral evidence. We address this point using fine-grained administrative voting data from Medellín, Colombia, where civic lists (grupos significativos de ciudadanos, in Spanish) provided a new path to enter institutional politics. We examine the electoral rise of Daniel Quintero, a political maverick who worked as senior advisor and vice-minister for the conservative national government but decided to create its own party brand and ran for mayor in Medellín by pursuing young voters through progressive issue appeals and digital campaigning during his successful 2019 bid. How did this change young voters' behavior? Our identification strategy exploits a distinctive feature of Colombian electoral administration: new voting booths are created within polling stations once registered voters exceed 360, generating systematic age variation across booths nested within the same station. This rule allows us to compare youth-heavy and older-skewing electorates while holding constant hyper-local political context and party supply. Quintero's vote share was roughly twice as large in youth-heavy booths as in older ones. These gains are not driven by differential turnout. They reflect a shift in voting away from established parties, most notably the conservative parties. Additional heterogeneity analyses and qualitative fieldwork help us understand how youth-oriented outsider campaigns redraw the competitive landscape in ways that survey data may not precisely capture. By leveraging fine-grained administrative electoral data, this study provides rare behavioral evidence on how populisms can reshape electoral competition at the ground level.