In studies of dementia, the age at onset (AAO) of the disease is often described without indicating how it was obtained. We used the “CAMDEX algorithm,” an ad hoc procedure, to compute the AAO of dementia form the CSHA database. An AAO could be calculated for 983 of 1,132 subjects with dementia. A similar procedure (the “clinical algorithm”) was used to calculate a second AAO, which was compared to that obtained by the first algorithm. The CAMDEX and clinical algorithms produced mean AAOs of dementia of 80.5 years (SD = 9.2 years, n = 983) and 79.6 years (SD = 9.7 years, n = 829), respectively. The sample correlation coefficient between the CAMDEX and clinical algorithms was .899 while the intraclass correlation coefficient, ICC(2,1), was .898. This method could prove useful for researchers using the CSHA data who need an AAO for those subjects with dementia.