It is a thought that crosses many middle-aged minds when a word is forgotten or a set of keys misplaced: Is this a fluke, or the first sign of dementia?

"Most of us will experience some cognitive changes with age," said Molly Wagster, chief of the behavioral and systems neuroscience branch of the National Institute on Aging (NIA), who likens such mental changes to the gradual slowing of a marathon runner's race times with advancing years.

The ability to call up words is one of the first things to slip: "You might find it more difficult to recall someone's name or the name of a book you read or favorite movie. Eventually, you will remember it, but it takes a little longer," Wagster said.