Abstract Psychiatric illnesses can affect the social transitions of adolescence and young adulthood, such as completing education and entering working life and relationships. However, associations between earlier onset age and long-term outcomes among those with early-onset psychoses (EOP) are unclear, as are the long-term outcomes of EOP compared to non-psychotic disorders. We used national register data of the Northern Finland Birth Cohort 1986 to detect persons with EOP and other early-onset psychiatric disorders. The long-term clinical and work-family outcomes of persons with onset age before 18 years (n = 41 psychoses, n = 495 non-psychoses) or between 18–22 years (n = 61 psychoses, n = 377 non-psychoses) were compared. Individuals with the onset of psychosis between 18–22 years had significantly more unfavourable long-term outcomes when compared to those with psychosis onset before 18 years. Persons with psychosis onset before the age of 18 years had similar outcomes to those with non-psychotic psychiatric disorder onset before 18 years regarding educational level, marital status, having children, and substance use disorders. Individuals with EOP were more often on a disability pension compared to those with other early-onset mental disorders. Adjusting for sex, educational level and substance use only slightly diluted these results. Unexpectedly, later onset age of EOP was associated with worse outcomes. Those with psychosis onset between 18–22 years of age are in a critical period, which underlines the importance of investing on interventions in this age group. Further studies on the effect of the onset age on later outcomes in EOP are needed.