This cohort-sequential panel study on Dutch youths (N = 3394) and their parents examined the formation of party preference between age 12 and 25. Specifically, it aimed to pinpoint the most formative component and age in a multiparty context. Opinionation, stability and correlates were examined for three components of party preference: party identification, voting intention and left-right identification. Results revealed that most youths formed a preference at some point during their early life. The 6-year stability of party preference was already substantial during early adolescence and increased until early adulthood. Party preference became increasingly related to youths’ social characteristics and issue attitudes with age, but parents remained important. Whereas studies from two-party systems emphasized the importance of party identification, this study suggested that left-right identification may instead predominate the early formation, intergenerational transmission and life-course stability of party preference in the Netherlands. The most formative period was around age 18.