This study examined (1) emotional versus cognitive developmental trajectories and (2) the influence of age-extrinsic factors (i.e., sex and puberty). Using a cross-sectional design, adolescents (N = 252) divided into four age-groups (ages 13, 15, 17, 19) performed two versions of a mentalizing task, about emotions and actions, as well as the Tower task. First, performance on all tasks improved linearly into late adolescence (age 19). Thus no differential trajectories were found for emotional versus cognitive development. Second, girls outperformed boys in mentalizing speed regarding both emotions and actions. In boys, a later pubertal phase was associated with increased mentalizing speed after controlling for age-group.