<p>This cross-sectional study explores the age-prevalence of teacher-rated aggressive acts among 1-to-5-year-old children (<i>N</i> = 919, 49.8% girls) enrolled in 20 Norwegian Early Childhood Education and Care (ECEC) centers. Perpetration of physical-, verbal-, relational-, and object-related aggressive acts towards peers was examined as well as victimization by using the same four types of acts. In the literature, such acts have been studied both using the concepts of aggression and bullying. The prevalence of all forms of aggressive acts was higher among the 2-year-olds than the 1-year-olds. In the second year of life, more than 50% of the children used physical aggression at least 2–3 times a month. In children aged 3&#xa0;years and older, the age-prevalence of physical aggressive acts declined, whereas verbal and relational acts increased or stabilized. Destroying belongings of peers occurred at low rates in Norwegian ECEC centers. The age-prevalence curves and absolute percentage levels for victimization and perpetration were similar. Our results suggest a general picture of similarities between girls and boys rather than differences. Among the 1- and 2-year-olds, there were no significant gender differences in any of the four aggression types for perpetration and victimization. The prevalence rates for girls and boys in relational perpetration and victimization were similar across all age categories. A significant gender difference in the prevalence of physical aggression appeared in the 3-year-olds. More consensus in definitions and measurements is called for including more research on the relation between general aggression and bullying behavior.</p>