Walking into a room and forgetting what you went there for is one thing. Locking your keys in your car? Totally understandable. Our overworked brains can only take so much, and so we forget things. But at the onset of dementia, the small forgettings snowball into real cognitive impairment. And a new study published today in Neurology suggests that when the problem is serious, people can tell.

The researchers analyzed the data of more than 500 people aged 60 and older who participated in a longitudinal study. At the beginning of the study, all participants were “cognitively intact.” As time went on, researchers noted any subjective memory complaints the subjects had, and also kept track of who was diagnosed with clinical mild cognitive impairment (MCI) or dementia. Participants also agreed to donate their brains to be studied upon their death.

More than half of the subjects enrolled in the study reported some change in their memories. The average age at which participants began to complain about memory problems was 81.5, an average of 8.3 years after they joined the study. For those who were later diagnosed with MCI, that transition took an average of 9.2 years.

About one in six of the participants developed dementia, and 80 percent of those who did first reported subjective memory complaints. Among those who died (about half of the participants), even people who were clinically unimpaired showed more dementia-like brain pathology in their autopsies if they’d had subjective memory complaints in life.

Though the study is limited by the fact that many participants were predisposed to dementia (they had a family history, or carried a gene that put them at risk), it does suggest that when someone notices changes in their memory, that’s a strong risk factor for future cognitive impairment. But, the researchers emphasize the long average time span between memory complaints and an actual diagnosis—valuable time for patients and physicians to attempt interventions that may prolong the good years.