Tom Brady's decline will come eventually. The question of the Pats' off-season will be: When?

Tom Brady turns 40 this summer, and history tells us a quick decline is inevitable. We just don't know how long from now that will be—and neither do the Patriots, who won't be able to keep Jimmy Garoppolo forever.

HOUSTON — Tom Brady arrived late to his Tuesday media availability, presumably because he was busy deflating footballs. I’m kidding (well, it’s true he was late), but I just wanted to squeeze a completely gratuitous Deflategate reference in before we move on to a far more interesting and consequential story about Brady: The endgame.

Brady will turn 40 this summer. You would never know it from watching him play football. He is as good as he has ever been. This was his average game this year: 24 for 36, 296 yards, 2.3 touchdowns, 0.3 interceptions. If he had played all 16 games (he missed the first four for reasons I can’t remember—I think he was riding a dead horse), he might have beaten out the Falcons’ Matt Ryan for league MVP. (The award hasn’t been announced yet, but will likely belong to Ryan.)

Brady’s numbers are impressive, but what’s just as impressive is how the Patriots QB produces them. In an age when NBA players in their prime skip games to rest, there is no indication that Brady is pacing himself.

I asked receiver Julian Edelman if he has seen any concessions to age for Brady, and he says, “It sure doesn’t look like it. He handles it better than anyone I’ve ever seen, that’s for sure. … What shocks me is the level of consistency, how he consistently goes through his routine each week. He continually does the same thing that he feels makes him his best.”

The Boston Connection: Matt Ryan's BC glory days in the city Tom Brady already owned

Brady’s personal fitness routine is all-consuming and well-documented. When he wants a treat, he eats avocado ice cream. His lifestyle has become a brand. He believes he has the answers to questions that we thought had no answers.

Defensive back Patrick Chung unwittingly gave an amazing ode to Brady’s longevity when he said, “I’ve only been with him eight years, so I don’t know what the other nine years were like. It’s the same Tom I’ve seen, always taking care of himself.”

At various times, Brady has said he wants to play until he is 45, until he is 49, or as long as he can. Brady is like most athletes: He wants to play until he can’t, or at least, until he can’t play to anywhere near his standard. We just don’t know how long that will be.

A year ago, Peyton Manning won the Super Bowl in his final game. If Brady wins his fifth championship, there is no chance he retires. Manning looked finished; nobody wanted to see him try to play this year. Brady is still in his prime.

The Big Interview: Playing days behind him, Michael Vick now rooting for his former team

But history tells us that whenever Brady declines, it will be quicker than we expect. Two years ago, Manning put up virtually the same numbers Brady did this year. Manning’s average 2014 game: 24 for 37, 295 yards, 2.4 touchdowns, 0.9 interceptions. He’d had multiple neck surgeries and his arm strength wasn’t the same, but he was still an elite quarterback. Then the 2015 season started, and it was clear almost immediately that Manning had lost it. He slogged through the year as a below-average quarterback and won that Super Bowl mostly because of his defense.

Manning’s decline was shocking but should not have been. At 40, Brett Favre’s average game: 23 for 33, 262 yards, 2.06 TDs, 0.4 INTs. At 41, Favre had a Manning-like decline but without the defense to carry him, and he retired.

These comparisons are imperfect—a quarterback’s raw numbers can be misleading, and Brady is clearly in better physical shape than Favre and Manning were near the end, even when they were playing well. But Brady will decline. The question is when, and how quickly.

Sports Illustrated's 52 Super Bowl Stories

And this brings us to the Patriots. Bill Belichick is famously cold in his personnel calculations. When he sees you start to go, he sends you on your way. One assumes that calculation will be at least a little different with Brady, partly because of the Kraft family’s affection for Brady, and also because Brady, more than any player in NFL history, deserves a chance to bounce back from a bad season whenever he finally has one.

But the Pats can’t just keep heir apparent Jimmy Garoppolo on the bench for as long as they’d like. Garoppolo was very impressive when he filled in for Brady last fall. He is due to be a free agent after the 2017 season. Historically, Belichick would flip him to another team this winter for a high draft pick or two, the way he did with Matt Cassel and Ryan Mallett at this stage of their careers. The difference is that Garoppolo looks better than Cassel and Mallett did, and Brady was not about to turn 40 then.

We are about to find out how Belichick really feels about Garoppolo, but also how much he really agrees that Brady can play another five to 10 years. Belichick is a master at finessing these situations—getting players to take below-market deals and knowing exactly when to move on. Brady has shown he’s willing to take much less than his market value, but will he do it so that Belichick can keep his successor on the roster? Is Garoppolo willing to sign an extension with the Patriots without knowing if he will ever actually play? And how much will Garoppolo actually fetch in the trade market?

These are not questions for this week, and there are so many variables that even Belichick may not know where this one is headed. But it will make for a fascinating off-season for the best player and team of this generation.