Jerry Yang is no longer among those intrepid youngsters who are CEOs of very big companies, though he was long the most famous and powerful of them. It wasn't a birthday that took him out of the mix, either. Yang, 39, was just shunted aside at Yahoo! for the more venerable Carol Bartz, 60, formerly of Autodesk. Who does that leave? Some 21 CEOs who are under 40 now run companies worth at least $500 million, our research shows.

Francisco D'Souza, 39, heads what is now the largest public company run by a 40-or-under CEO, though his $5.7 billion Cognizant Technology Solutions is dwarfed by the $16.9 billion Yahoo!. D'Souza has been an officer at Cognizant for 11 years now, chief executive since 2007, and chief operating officer since 2003, when he was a mere 33.

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Amphenol's R Adam Norwitt and CNX's Nicolas J DeIuliis run the second and third largest public companies on the list. Norwitt is one of 10 who run tech-oriented companies. That sector, always associated with youthful genius and entrepreneurship, has far and away the largest representation. Energy and health care each have just three companies on the list.

Joshua G James, 34, of Omniture, is the youngest of all these CEOs. His business is worth $750 million. James was one of Omniture's founders, and in his last reported year of pay, he made more than $2 million. Not bad for a guy born after all the troops came home from Vietnam.

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DynCorp International's William L Ballhaus and WellCare's Heath G Schiesser will both hit 41 soon and retire from the realm of these daring youths. Will anyone replace them, or will there be increasingly fewer young CEOs at large corporations as we move farther away from the tech boom? That has been the trend so far.

The most experienced and seasoned old CEOs have to make hard-nosed decisions and endure intense scrutiny during tough times. Young CEOs may find themselves under an even brighter spotlight, thanks to their supposed inexperience. But at least they have plenty of time ahead of them to correct any errors -- and possibly move on to even bigger things.

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