A new study of Nobel Prize winners and great inventors suggests top innovators are older today than they were a century ago.

"I find that the age at which noted innovations are produced has increased by approximately 6 years over the 20th Century," says Benjamin Jones of Northwestern University. "Innovators are much less productive at younger ages."

Why the change?

Jones figures that the accumulation of knowledge over time -- all that stuff that most of us don't know -- means even great minds need to spend more time educating themselves before they can make a breakthrough.

That means the squeeze is on to produce great work, the survey shows.

"Individual innovators are productive over a narrowing span of their life cycle, a trend that reduces -- other things equal -- the aggregate output of innovators," Jones writes.

The research was presented this month as a working paper by the non-profit National Bureau of Economic Research.

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