The age at first Alzheimer's dementia diagnosis (dx) variable represents the age at the first cycle where an Alzheimer's dementia diagnosis was rendered. This is calculated using the variables age at visit and clinical diagnosis summary (value = 4 or 5). This is the best approximation of "age at onset of Alzheimer's dementia" available, as most participants are seen on a yearly basis. This measure is not available for participants that were demented at baseline cycle.