Background In the national study of multiple registers in 2000, the average prevalence of intellectual disability (ID) was 0.70%, with marked differences by age group (range 0.38–0.96%) – what are these differences in detail, and can they be understood? Method This study was based on two national health registers and six social benefit registers. Prevalence of ID was calculated by 1‐year age cohorts. Results The multiple register prevalence of ID increased steadily from 0.20% in the first life year to 0.74% (male: 0.90%, female: 0.58%) at 10 years. For boys, the rate fell to 0.71% at 11 years. For both sexes, a steady increase was noted in the distribution up to 40 years (male: 0.84%, female: 0.73%), followed by a sharper increase to the maximum prevalence (male: 1.19% at 48 years, female: 1.05% at 50 years). At the pension age of 66 years, a sudden drop to 0.49% occurred for men and women. Different registers gave very different age distributions. Conclusions By examining the data by 1‐year age cohorts, and by understanding the role of each register, it could be deduced that a proportion of cases in younger age groups is lacking, and a remarkable proportion of elderly ID persons is missing from the pooled data. The findings were more difficult to interpret, if the data were grouped into bigger age groups.