A ten-year study involving 2,092 older adults (average age 76) has found that people tended to lose awareness of memory problems two to three years before the onset of dementia.

Being unaware of your own memory problems is common in dementia, but previous research has focused on those already diagnosed with dementia. In this study, participants had no cognitive impairment at the beginning of the study.

Overall, subjective memory ratings taken annually were modestly correlated with performance (only modestly — people tend not to be that great at accurately assessing their own memory!), and this awareness was stable with age. However, in the subset of those who developed dementia (239 participants; 11%), this awareness started to deteriorate an average of 2.6 years before dementia was diagnosed (after which it dropped rapidly).

In a subset of those who died and had their brains examined (385 participants), a decline in memory awareness was associated with three pathologies:

• tau tangles
• gross cerebral infarcts
• transactive response DNA-binding protein 43 pathology (TDP-43 is a protein involved in transcription, the first step in producing proteins from genes; mutations in the gene that produces TDP-43 have been linked to frontotemporal dementia and amyotrophic lateral sclerosis (ALS)).

There was no decline in memory awareness in those who didn't show any of these pathologies.

Those who were older at the beginning of the study were more likely to retain memory awareness longer, perhaps because they were more alert to memory problems.

http://www.theguardian.com/society/2015/aug/27/dementia-sufferers-start-losing-memory-up-to-three-years-before-condition-develops-us-study