Numerous studies have found that male schizophrenic patients have earlier ages at onset than females. However, none of these studies have corrected the observed ages for known gender differences in the age distribution of the population. Using a pre-existing data set, we applied a non-parametric method to correct the male and female distributions of observed age at onset for sex-specific age distributions. The distributions of observed age at onset indicated earlier onset among males. After correction, the age-at-onset distributions shifted toward older ages, but the difference between males and females remained statistically significant. Thus, gender differences in the age at onset of schizophrenia are not artefactual.