We've all had the experience of forgetting something we've known for years: a friend's name, the title of a favorite song, where we left our keys, etc. Put those common occurrences together with knowing an elderly person with dementia and a brief memory lapse becomes a frightening glimpse of future mental deterioration.
Head for the gym, the mental exercise gym, as August 9th's article Stay Mentally Sharp describes the benefits of mental exercising.