In a Limited-Duration Prevalence session, the Age at Prevalence Date (Std Pop, Pop, Case Files - Calculated) box on the Selection tab contains only the Age at Prevalence Date (Calculated) variables. The groupings defined in this variable are the same as the database's age recode variable. The age recode variable differs from one database to another based on the associated population and standard population data. For example, some have 18 age groups while others have 19. User-defined versions of this variable, if any exist, will be shown in a folder named "Calculated".
This is called a calculated variable because the age at prevalence is not coded in the database. This value is calculated based on the selected prevalence date and either date of birth (if available) or age and date at diagnosis.