Data from two longitudinal studies of older adults (a nationally representative sample of older adults, and the Alzheimer’s Disease Neuroimaging Initiative) has found that a brief cognitive test can distinguish memory decline associated with healthy aging from more serious memory disorders, years before obvious symptoms show up.
Moreover, the data challenge the idea that memory continues to decline through old age: after excluding the cognitively impaired, there was no evidence of further memory declines after the age of 69.
The data found that normal aging showed declines in recollective memory (recalling a word or event exactly) but not in reconstructive memory (recalling a word or event by piecing it together from clues about its meaning, e.g., recalling that “dog” was presented in a word list by first remembering that household pets were presented in the list). However, declines in reconstructive memory were reliable predictors of future progression from healthy aging to mild cognitive impairment and Alzheimer’s.
 Dual-retrieval models and neurocognitive impairment. Journal of Experimental Psychology: Learning, Memory, and Cognition. 40(1), 41 - 65.(2014).