There isn't one.
Manuel should remain as much of a fixture around Citizens Bank Park over the next few years as the supporting beams and the Phillie Phanatic. Goodness knows he has the resume, with five National League East titles, two pennants and a World Series championship.
Age, age. Did somebody mention age? Since Manuel is several decades older than the likes of Bryce Harper, for instance, are folks actually saying age is a factor here?
Jack McKeon isn't among them.
"Heck, I can still manage right now, and I'm 82," McKeon, the legendary baseball man of 63 years, said over the phone from the spring home of the Miami Marlins in Jupiter, Fla. He took a break from his daily two-hour workout to discuss the silliness of those fretting over how long the 69-year-old Manuel can remain effective running the Phillies.
It's silliness based on short-sightedness, suggested McKeon, who then added, "It's the same way [throughout society]. As soon as you get to be 65, they want to put you out to pasture. They say you can't communicate today. They say you're not hip with all of these guys.
"Maybe you don't have your iPod or your video games like the younger folks do, but here's what I don't understand: Why would you want to penalize experience?"
Here's partly why: following tradition. More specifically, bad tradition. If you're running a baseball team, and if you sift through history, you'll discover the unspoken age limit for managers is 70, and managers have complied -- either by choice or by force.
Though he interrupted his retirement this year to manage Team USA in the World Baseball Classic, Joe Torre was 70 when he retired after the 2010 season. Future Hall of Famers Bobby Cox and Tony La Russa retired in recent years at 69 and 67, respectively. And though he went on to manage the Mets, Casey Stengel was forced out by the Yankees in 1960. "I'll never make the mistake of being 70 again," he said.
There have been huge exceptions, though, starting with Connie Mack, who retired at 87 after managing the Philadelphia Athletics for 50 years through 1950, shortly before his 88th birthday. He won five World Series championships and nine American League pennants, but he also finished last 17 times.
It didn't hurt Mack's job security that he either was owner or part-owner of the Athletics during that stretch.
But let's discuss other exceptions.
"Let's go back to one of the premier franchises in the game today, the Dodgers," McKeon said. "They had two managers [Walter Alston for 23 years and Tommy Lasorda for 20] during all of that time [1954-96], and they were smart enough to know that if the club didn't do well, it wasn't wise to ax the manager all the time. Change the players.
"Instead, teams get too jumpy. 'This guy is too old. We gotta go out there and get somebody young.'"
Despite their longevity, Lasorda retired at 68 and Alston at 64, but then there was McKeon, who worked for a Marlins team that decided a manager's age was just a number.
McKeon became the oldest manager to win a World Series, at 72, when he led the 2003 Marlins past the New York Yankees. He was 80 when he returned in 2011 to manage the Marlins on an interim basis.
So, what the extremely youthful Manuel -- at least compared to McKeon -- told The Associated Press this week deserves a yawn.
"I still want to manage," Manuel said, expressing his wish to remain with Philadelphia for a number of seasons beyond this one. "I'm not ready for somebody to tell me to go home. I'm not ready to quit managing. I'm not ready to get out of the game."
For one, Manuel has won more games as a manager than anybody for a Phillies franchise that lost more games during the 20th century than any other pro sports franchise in North America. He also is one of just two Phils managers to own a World Series ring.
If that isn't enough, after Manuel led the Phillies to consecutive second-place finishes during his first two years, he helped them collect those NL East titles, pennants and that 2008 World Series championship.
The only reason Manuel's team slipped last year was a crushing number of injuries, but they still finished 81-81.
As for whether Phillies officials want Manuel around for all of 2013, 2014 and beyond, general manager Ruben Amaro Jr. said, "We'll see what happens at the end of the year and go from there."
McKeon sighed over the phone.
"He's a good manager. He's been very successful, and he's been great for the city and great for that franchise," said McKeon, who was Manuel's first Spring Training manager when Manuel joined the Minnesota Twins organization as an outfielder in 1963. "Why would you have to wait until the end of the season to see how he does before you decide to give him another contract?"