India, April 18 -- * Joan of Arc was in her teens when she led a platoon into battle.

* Galileo was 19 when he made some of his most dramatic discoveries about the density of matter.

* Newton was 23 when he had his Eureka moment about gravity.

* Mozart was touring Europe and performing before royalty by the age of six.

These were prodigies, certainly. But given how low life expectancy was until the modern era, researchers now believe that a large share of humanity's most dramatic discoveries-harnessing fire, the wheel, smelting, and plenty more - were likely made by people who were quite young.

Necessity drove invention, and necessity meant there were few rules about the burdens young people could carry.

The rules that society has e...