This study examined the combined effects of age-based self-categorization and aging expectations on cognitive performance in a clinical context. An experimental study manipulated older adults' salient self-categorization as Younger or Older, as well as expectations that aging involves a specific memory decline versus generalized cognitive decline. Memory and general ability tests that are typically used in dementia screening were then administered. As predicted, self-categorization as Older dramatically reduced performance, but the measure on which this effect was revealed depended on aging expectations. Participants who self-categorized as Older and expected memory to decline performed worse on memory tests. Conversely, participants who self-categorized as Older and expected widespread cognitive decline performed worse on the general ability test. The clinical implications for the latter group were profound, because 70% met the diagnostic criterion for dementia, compared with an average of 14% in other conditions. The importance of self-categorization processes when interpreting performance on tests used to diagnose dementia are discussed.