There is an unwritten rule in the NFL personnel community about aging quarterbacks, and it has held up with remarkable accuracy over the years. Over the decades, in fact.
With few exceptions, even the greatest NFL passers don't last beyond age 38.
The list of great quarterbacks who succumb at this age is as remarkable as it is long:
Want to go further back?
There are some notable exceptions: Brett Favre lasted until he was 41. Vinny Testaverde made it to 44. Same with Warren Moon. But the general line of demarcation is 38.
Which brings us to Peyton Manning. Guess how old he is?
He is 38.
And it is time to walk away.
A year after producing the greatest regular-season numbers of his career and getting the Broncos to the Super Bowl for the first time since Elway's final season in 1997, Manning's wondrous career has hit the wall.
He flamed out the final month of the regular season, with only three touchdown passes and six interceptions in his last four games, including that four-interception meltdown in the rain in Cincinnati that first made you realize this could be it for him.
And in a playoff matchup against Andrew Luck, his successor in Indianapolis, Manning was unable to show the kind of mastery that defined his career.
One play offered a definitive reminder that his time is up:
Trailing 24-13 with 3:04 to play, on fourth-and-8 from the Broncos' 46, Manning chose not to throw downfield but instead dumped it off to running back C.J. Anderson, who was tackled inches short of a first down. The Broncos never got any closer, and Manning walked off the field after shaking hands with Luck, his head lowered and perhaps his mind coming to grips with the idea that this would be his final moment as a player.
Manning hedged when asked if he would consider retirement.
"I can't answer every what-if circumstance," he said. "What if you're not as healthy? What if certain coaches leave? I can't answer every what-if situation. I think I'll have to take some time to see how I feel, see how I feel physically."
His health is surely a big concern. ESPN reported Monday that Manning played the final month with a torn quadriceps. The extent of the injury is unclear because Manning never missed any significant playing time. A complete quadriceps tear usually prevents running and requires either surgery or extended rest.
Last year, throughout his run to the Super Bowl, Manning was unequivocal in his answers about retirement: He was coming back in 2014. And even after his woeful performance against the Bengals on Monday night in Week 16, Manning said he expected to return next season.
But CBS announcer Jim Nantz said near the end of Sunday's game that Manning had softened his stance. "He actually told us [in a production meeting] that 'I'm gonna wait and see; I can't really in my heart tell you anything for sure right now,' " Nantz said. " 'My intention is to play, but things have changed since I last said that.' "
This time he understands that time has caught up with him.
Maybe he gets away for a few weeks and decides he feels good enough to give it one more shot. But then he runs the risk of his skills eroding further, and I don't sense that Manning is willing to allow himself to become a pathetic figure just because he can't fathom a life without playing football.
Manning was listless Sunday. His arm strength just wasn't there and his body language was just as disturbing. Even when the Broncos were within range of the Colts in the second half, Manning's expression flashed more resignation than determination. At a time when he should have shown defiance, he looked demoralized after the Colts repeatedly stopped him.
Maybe Manning knew this was it, that he just physically can't do it anymore. Maybe he understood what so many who have come before had to face. What Broncos Hall of Famer Elway, who brought Manning to Denver in 2012, realized when he walked off the field one last time -- even if he walked off as a Super Bowl champion at age 38.
Elway, the Broncos' general manager and vice president of football operations, soon will sit down with Manning to discuss his future. No one better knows what Manning is going through than the former quarterback, who should tell Peyton what he needs to hear: It's time.