The second half of the twentieth century has been characterized by substantial changes in demographic behaviors. Among these transformations also the process by which adolescents and teenagers transition to adulthood has changed greatly in many countries of the Western world. All the events of the transition to adulthood have been delayed and life course trajectories became more diverse. There are some aspects concerning the mentioned changes that have not been extensively studied in the literature. This dissertation is a collection of three papers that have the aim to investigate these neglected aspects concerning life course trajectories of young adults. In particular, the first two papers look at trends over time in the achievement of economic independence, a crucial event in the transition to adulthood that has not received enough attention so far. The first paper is a cross-national comparison describing the situation in six different developed societies. The second paper studies only the United States, going back to the 1970s and tracing changes over time until 2007. The third paper, instead, focuses on the role of parental social class in the transition to adulthood. The exact mechanisms by which socio-economic status affects the transition to economic self-sufficiency and family formation are largely unknown. A better understanding of these issues can highlight additional information to understand why and how the transition to adulthood has changed in the last five decades. Analyses were carried out using survey data from the Luxemburg Income Study (LIS), the National Longitudinal Surveys (NLS, NLSY79, NLSY97), and the Multipurpose ISTAT (FSS 2003). A first main finding of this study is that the transition to economic independence has been delayed together with all the other events of the transition to adulthood. This process has occurred in all developed Western countries even if with some differences. A second finding is that parental social class can explain some of the variation in life courses, and that a higher social class is associated with a postponement in the transition. Also the role of family background, however, differentiates based on welfare state regimes, institutions, and the strength of family ties.