Here's a fact: Due to his age, Warren Buffett is not qualified to be a director of three banks – Bank of America, U.S. Bank, and Wells Fargo – in which he has substantial investments.

U.S. Bank retirement policy requires all directors to retire at the age of 72.  Wells Fargo and Bank of America have an interesting twist to their retirement policies.  Each has mandatory retirement, but allows the bank to retain existing directors if the board's nominating committee deems it “in the best interests of the company” to keep the retirement-age director.

Fifth Third Bank also has a mandatory retirement age for directors. William Isaac, the former Federal Deposit Insurance Corp. chairman, has been a Fifth Third director since 2010.  As he noted in his Feb. 18, 2014 op-ed for American Banker, he will step down from his post in April when he turns 70.

Proxy season is nearly upon us. Perhaps this is a good time of year to ask why some banks require older directors to step aside.  Since this practice is so common, it must be good corporate governance.

Or is it? 

Warren Buffett apparently thinks older directors are just fine for Berkshire Hathaway, one of the most admired companies in the world. Of the company's 13 directors, six including Buffett, are in their eighties.

In conducting research for my book, Broke: America's Banking System: Common Sense Ideas to Fix Banking in America, I examined the performance of 382 U.S. banks with assets below $10 billion that have been publicly traded since 2002 and remained in business through early 2012. The vast majority did not serve their shareholders well through the financial crisis: the average bank's stock price fell 43% during those 10 years.

Of the 382, shareholders in only 30 saw stock price appreciation for each of the one-, five-, and ten-year periods ending Jan. 31, 2012.

Additionally, of those 30 banks, 20 had directors on their boards now deemed too old to serve on the Fifth Third board.

These 20 top-performing banks – based on shareholder performance – had a combined 59 directors 70 years of age and older. Almost half were at least 75 years old. (The source for all board data is Morningstar, 2012.)

Eight of these 20 banks were chaired in 2012 by a person at least 70 years old. Three of those bank chairmen, like Warren Buffett, were octogenarians.

Texas is especially friendly to older directors. Four Texas banks were among the thirty top performing banks for shareholders from 2002 to 2012. Combined, they had 17 directors 70 years of age and older.  Seven of the 17 first became directors between 1982 and 1993, a time when more than 800 Texas banks and S&Ls failed. Directors and CEOs who survived such upheaval have long memories.

Only one publicly held U.S. bank's shareholders enjoyed stock price appreciation greater than 200% for the ten-year period ending Jan. 31, 2012. That bank, Arkansas' Bank of the Ozarks, was up roughly 800%. Oh, by the way, five of its directors were at least 71 years old in 2012. All but one remains on the board today.

Is it possible bank directors over 70 years of age are actually associated with safer, better performing banks?

If true, the explanation may be found in a book written in 2012 by a former Deutsche Bank derivatives trader. In The Hour Between Dog and Wolf: How Risk-Taking Transforms Us Body and Mind, author John Coates builds a case that bank trading floors are dominated by pedal-to-the-metal young men overloaded with caveman testosterone. The young male trader is the backbone of a Wall Street business long associated with flashes of brilliance and bouts of devastating losses.

Coates is unusually prepared to tackle the study of hormones and banking. In addition to his experience on a derivatives desk, Coates has a Ph.D. in economics and an M.D. in neuroscience. He suggests banks can throttle back trading risk by employing more middle-aged men and women who have fractions of the testosterone seen among younger traders.

You have to wonder what Coates might think of 75-year-old bank directors.

If banks that performed well for their shareholders from 2002 to 2012 have so many older directors, why would some banks force directors to retire once they hit 70 or 72 years of age?

Perhaps banks with mandatory retirement ages have evidence that old directors are ineffective. If so, in the interest of the safety and soundness of the banking industry, they should share their data so banks without mandatory retirement can evaluate whether they, too, should adopt the practice and avoid running into future problems.

As well intended as mandatory retirement may be, it can run counter to shareholders' interest. Great bank directors are rare. They are also invaluable to a bank's long-term success.

By any criteria, Isaac is well qualified to be a bank director. Surely Fifth Third believes so, having paid him over $1.2 million in his first three years on the board.

His bank's shareholders have been rewarded with performance better than peer banks.

Clearly, many well-managed banks enforce mandatory retirement of directors. But if skilled directors are lost as a result, does it really make sense? Cannot board-nominating committees find another way to address underperforming directors, irrespective of age?  And if a board wants a retirement policy, why not just do what Bank of America and Wells Fargo do and keep the option of delaying retirement ‘in the best interests of the company'?

Richard J. Parsons is the author of Broke: America's Banking System: Common Sense Ideas to Fix Banking in America, published in 2013 by the Risk Management Association. He spent 31 years at Bank of America.