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Background Many studies have evaluated the prevalence of intellectual disability (ID) by focusing on different ages during childhood and adolescence. Although the prevalence of ID is higher in older age groups, how cumulative prevalence increases, and what level it reaches before adulthood, remains unclear. Method We used Care Register for Health Care to retrieve information on individuals born in 1996-2007 with any of the inclusion diagnoses of ID (F7 group and/or aetiological diagnoses) for the period 1996 to 2013. The cumulative prevalence was calculated as percentages for every age based on Finnish population data. Results The registration of new diagnoses of ID continued steadily throughout the developmental years. The cumulative prevalence reached 1.19% by age 17.5 among those born in 1996. Later-born age groups appeared to receive their first ID diagnoses earlier in childhood. Those born in 1999 reached a cumulative prevalence of 1.21% already by age 14.5. Of all those with ID, 67% had an F7 diagnosis only, 42% had an aetiological diagnosis only and 9% had both diagnoses. Conclusions Cumulative prevalence of ID by year, until the age of 18, will provide a better estimate and understanding of the prevalence of ID than a point prevalence at any one point during the developmental years. [ABSTRACT FROM AUTHOR]