Adolescence and young adulthood are characterized by substantial sociodemographic, family, social, and personality changes that may influence loneliness. Although loneliness is a public health challenge, we know little about how loneliness develops during these periods. Our study addresses this lacuna by using 4-wave longitudinal data from 3,116 Norwegians aged 13 to 31 years, making use of questionnaire (key facets and correlates of loneliness) and register linkage information (midlife outcomes). Analyses revealed that when asking directly about feeling lonely and for emotional facets, loneliness increased from early adolescence to age mid-20s, whereas social facets of loneliness declined gradually and plateaued when people had reached their mid-20s. Several predictors operated consistently across loneliness facets, whereas others operated in facet-specific ways. To illustrate, perceiving one's parents as caring, having close friends, not leaving the parental home before age 18, and reporting more agency were each associated with less loneliness across assessment modes. In contrast, when asked directly, women reported more loneliness than men at all ages, whereas men reported more social loneliness. Finally, adolescents and young adults who reported feeling lonely and/or increased in loneliness were consistently at higher risk for disability and lower income in midlife, whereas other important midlife outcomes including education, labor market inclusion, and prescriptions of antidepressants exhibited facet-specific associations. Our study is the first to provide a comprehensive picture of loneliness development throughout the second and third decade of life and highlights the multidimensionality and multidirectionality of loneliness trajectories and correlates across adolescence and early adulthood.