A few years ago, Michael Ramscar, a linguistics researcher at the University of Tübingen in Germany, came across a paper saying that cognitive decline starts as early as age 45. He was 45 himself and felt he hadn't yet peaked. He remembers thinking: "That doesn't make sense to me; 99 percent of the people I look up to intellectually, who keep me on my mettle, are older than I am."

The paper concluded that a person's vocabulary declines after age 45, and that finding really made no sense to him. The researchers were trying to measure how quickly people remembered words, he says, without even considering the quantity of words they had stored in memory.

Ramscar began to wonder who has