It is well established that at the population level the age-specific incidence of schizophrenia is highest in the early 20s and decreases with age. We present new findings showing that this is not the case when we assume that only part of the population is susceptible. The study population consisted of birth cohorts born in Finland from 1950 to 1968. These were identified from the Population Register Center, which provided the data as multidimensional tables with sex, and year, month, and place of birth as marginals. Each cohort was followed-up until 1991, up to age 40. Data analyses were based on a compound Poisson statistical model, which assumed that only part of the population is susceptible to develop schizophrenia. The method detects the proportion of susceptibles in population level, but does not identify susceptible individuals. The model was able to reveal the basic hazard distribution, which showed increasing risk of developing schizophrenia with higher age at the individual level among the susceptible part of the population. The increase in risk was more rapid among males, but the susceptible part of the population was of similar size among both sexes. The proportion of susceptibles seemed to be lower in younger cohorts. We conclude that when an individual belongs to the susceptible part of the population, his/her individual risk of developing schizophrenia increases with age, at least up to age 40. This increase is more rapid among males. The reasons behind the lower proportion of susceptibles in younger birth cohorts, as well reasons for differences in risk between sexes remained unclear and need more research.