IBM (NYSE:IBM) is in for another C-suite change. Longtime CEO Sam Palmisano retired at the beginning of 2012, replaced by sales and marketing chief Ginni Rometty. Now, the longest-tenured chief financial officer in IBM's century-plus history will retire at the end of December.

IBM CFO, Mark Loughridge

Retiring IBM CFO Mark Loughridge is the man behind today's falling IBM shares. He will be missed. Image source: IBM.

CFO Mark Loughridge is turning 60. An IBM spokesman calls it "the customary retirement age," which might be true for IBM. Palmisano retired at that age, for example.

Most Americans retire between the ages of 65 and 67 in order to obtain full government retirement benefits.

Corporate policies on retirement age aren't unheard of: Fellow tech giant and Dow Jones (DJINDICES:^DJI) component Intel (NASDAQ:INTC) insists on replacing CEOs when they hit the age of 65. Current chief executive Brian Krzanich could last until 2025 without breaking Intel traditions. Intel shares have underperformed the Dow during Krzanich's six-month reign, just as IBM stock has disappointed investors under Rometty. Growing pains or executive-suite downgrades? Only time will tell as the new leaders take a couple of years to implement their strategies.

But never mind government rules or industry standards. IBM is certainly free to set its own rules, standards, and traditions. If 60 is the hard cutoff age for IBM leaders, incoming CFO Martin Schroeter could stick around for a while. The former IBM treasurer and current head of global financing operations is only 49 and could actually beat Loughridge's record-breaking tenure (just a few months shy of 10 years) before retiring at 60.

The CFO succession may be planned in detail, and Schroeter does come with credentials similar to Loughridge's pre-CFO resume. But Loughridge leaves some mighty big shoes to fill.

The soon-to-be-former IBM executive was recently chosen as America's finest CFO by The Wall Street Journal, which cited his investor-friendly long-term road map and near-universal respect from his peers. When CNNMoney selected a "dream team" of business leaders, Loughridge was the CFO. He's "the epitome of a team player" and a masterful organizer, CNN said. In a poll conducted by Institutional Investor a couple of years ago, Wall Street analysts pegged Loughridge as the best CFO in America. He's the man taking most of the questions on IBM's earnings calls, and he often serves as IBM's public face to the world.

IBM investors are feeling the pinch as Big Blue gets ready to replace one of the top performers at his position. It's kind of like the Chicago Bulls when Jordan retired, or the Rangers preparing for Gretzky's final game.

IBM shares fell 1% on the news, missing out on the Dow Jones 0.64% jump higher. If IBM had traded sideways instead of down, the Dow would have been spared a 12-point drop from IBM alone. It's fair to say that Mark Loughridge's exit is market-moving news.

Of course, Warren Buffett famously says that truly great companies could be run by a ham sandwich and be no worse off for the wear. Buffett owns an $11 billion stake in IBM, so Big Blue probably qualifies for the Ham Sandwich Awards. I'm not saying Martin Schroeter is a piece of meat between two slices of pumpernickel, but IBM investors should relax a bit. IBM will do just fine even if Schroeter never makes it into the CFO hall of fame.

It's just a bonus if he does. He's got about 10 years to make it happen.