Scientists researching dementia say the time for people to be concerned is when they stop noticing memory lapses.
Experiencing "senior moments" may be a good sign rather than a cause for concern, research suggests.
The time to worry is when you begin to stop noticing memory lapses, scientists have shown.
A study found that people with dementia tend to lose awareness of memory problems two to three years before the condition develops.
"Our findings suggest that unawareness of one's memory problems is an inevitable feature of late-life dementia, driven by a build-up of dementia-related changes in the brain," US lead researcher Dr Robert Wilson, from Rush University Medical Centre in Chicago, said.
"Lack of awareness of memory loss is common in dementia, but we haven't known much about how common it is, when it develops or why some people seem more affected than others.
"Most studies of memory unawareness in dementia have focused on people who have already been diagnosed. In contrast, this new study began following older adults before they showed signs of dementia."
The team tracked the progress of more than 2000 individuals with an average age of 76 who were free of dementia at the start of the study.
Over a period of 10 years, they were given annual tests of memory and thinking ability, and were also asked how often they had trouble remembering things.
For the 239 volunteers diagnosed with dementia, memory awareness began to drop sharply an average of 2.6 years before they developed symptoms.
Several years of memory decline followed.
"Although there were individual differences in when the unawareness started and how fast it progressed, virtually everyone had a lack of awareness of their memory problems at some point in the disease," Dr Wilson said.
The findings appear in the journal Neurology.