That tradition helps explain why IBM CEO Sam Palmisano and why , Virginia Rometty, 54, may have just six years to leave her mark on the company.

Palmisano isn't stepping down because of apparent displeasure by the board, of which he is chairman. He is leaving after having set a strategic direction for the company through 2015.

But there is a logic to a relatively early retirement by a CEO, and not just at .

The average age of a departing CEO is 61, according to The Conference Board, in a report this year on CEO succession. It calculated this figure after studying CEO turnover among companies listed on the Standard & Poor's 500 ranking of the largest public corporations.

That average age represents both retirements and those CEOs who, for one reason or another, were forced out of their jobs, likely in their 50s, according to Jason Schloetzer, an assistant professor of accounting at the McDonough School of Business at Georgetown University and an author on the study.