Objective: mental illness is the leading cause of sick leaves and disability pensions in many developed nations. In Finland, 51% of disability pensions were caused by mental illness in 2017. We investigated how age of onset (AOO) of mental illness affects long-term employment outcomes in prospective general population based Northern Finland Birth Cohort 1966 (n=11234).Methods: for people with schizophrenia (SZ, n=137), affective psychoses (AP, n=100), and non-psychotic affective disorders (NPAD, n=659) onset age was analyzed (quartiles for each diagnosis) from register data. A yearly employment rate for years 1979-2015 was calculated for the AOO groups for each diagnoses.A total employment outcome was created by calculating a mean employment rate for the whole follow-up time (1983-2015). A Kruskal-Wallis test was performed to statistically test the total employment outcome between AOO groups. Cohort members without psychiatric diagnosis were the healthy peers.Results: in psychotic disorders, especially in schizophrenia, the employment rate is low compared to other diagnoses and peers. People with an earlier onset tend have poorer employment rates than the ones with a later onset in all diagnoses. A Kruskal-Wallis test showed that there was a statistically significant difference of total mean employment between AOO groups in all diagnoses (p<0.05).Discussions: Early onset mental illness results in poorer employment in all the included diagnosis. The difference between the AOO groups in the annual employment rate is often in the early stages of work life and evens out towards the end of the follow-up period. This is especially true in psychotic illnesses. In non-psychotic illnesses, the differences are visible during the whole follow-up time, but to get a better understanding, a longer follow-up is needed as onset to these illnesses happens later than in psychotic illnesses. In all of the included diagnoses there is a statistical difference in the total employment outcome between AOO groups. Thus, the overall costs to societies are greater the earlier the onset happens. These results can guide in directing employment and social rehabilitation efforts towards the early onset patients.