One of the big questions that could swing the entire 2019 NFL season has few answers in the way of historical evidence: How will Tom Brady and Drew Brees (and, to a lesser extent, Philip Rivers) fare as they hit ages when few — if any — quarterbacks have ever been good before?

Brady turned 42 a week ago and Brees hit 40 in January. (Rivers turns 38 in December.) Both were very good last season, with each finishing among the league’s top eight QBs in numerous statistical categories including ESPN’s Total Quarterback Rating, Football Outsiders’ Defense-adjusted Yards Above Replacement and Pro-Football-Reference’s Approximate Value — where each posted a score of at least 14.

Speaking to the last figure, it was the first time since 1960 that three QBs aged 37 or older each produced that much AV in the same year. Being an old passer has never been so good. But no matter how much water Brady drinks, Father Time will get his ‘W’ eventually. Brady, Brett Favre, Warren Moon and Vinny Testaverde are the only QBs to ever put up 10 or more AV in a season at age 40 or 41, and no QB in history has ever recorded more than 5 AV at age 42 or older.

So, given that we know the end will come sooner or later… what will that decline look like for this current generation of aging quarterbacks? Will it come quickly — vindicating Max Kellerman — or will Brady and Brees just gradually lose effectiveness over time, while simultaneously easing into a reduced role?

In that regard, we can turn to history for at least some clues. Of the 29 retired QBs who had at least 10 AV in a season at age 36 or older, 12 saw a gradual decline after their final double-digit AV campaign, posting at least 5 AV in another subsequent season. (That’s not a very good number for a starting quarterback — it’s quite mediocre, in fact — but it’s not the mark of a complete disaster, either.)

They eased down the hill… Among QBs with 10+ Approximate Value in a season at age 36 or older, those who had at least one more 5+ AV season after their final 10-AV season, 1960-2018 Last 10+ AV Season AV in N Seasons After Quarterback Year Age AV 1 2 3 4 5 6 7 8 Sonny Jurgensen 1970 36 15 1 3 4 7 — — — — Len Dawson 1971 36 12 8 3 6 4 — — — — Babe Parilli 1966 36 11 8 2 1 — — — — — Billy Kilmer 1975 36 10 5 5 1 — — — — — Jim Plunkett 1983 36 10 5 3 6 — — — — — Carson Palmer 2016 37 12 5 — — — — — — — Steve DeBerg 1991 37 12 2 6 — — — — 1 — Dan Marino 1998 37 10 7 — — — — — — — Doug Flutie 2001 39 11 0 5 2 0 — — — — George Blanda 1966 39 10 7 7 3 4 6 5 3 4 Brett Favre 2009 40 16 5 — — — — — — — Warren Moon 1997 41 12 5 0 1 — — — — — Quarterbacks must be currently retired to qualify. Source: Pro-Football-Reference.com

Even after the final good seasons for Hall of Fame QBs like Favre, Moon, Marino and Len Dawson, they still managed to eke out at least one more year of moderately effective play before either totally fading away or simply retiring. (Or having the worst final game ever.) These relatively gentle declines offer one model for Brady and Brees to follow as the aging curve begins to hit hard.

Moon and Favre are especially interesting test cases, since each had been very good as they approached 40 before suffering down seasons. (Moon played only eight games and posted 4 AV in 1996; Favre played all 16 games and had 12 AV in 2008, but struggled greatly with an injury down the stretch and had a below-average QBR of 46.0.) Both responded the following year with terrific performances, defying the effects of time, and then even hung on for another additional 5-AV season before the end truly came. A similar tale applied to Washington’s Sonny Jurgensen, who appeared to fall off a cliff at age 37 and played only 12 combined games across 1971 and ‘72. But Jurgensen returned to spot duty behind Billy Kilmer (who would later join this list himself) in 1973, then mustered a very good year as a part-time starter in 1974 before retiring for good.

And longtime Oilers and Raiders QB/kicker George Blanda is such an outlier that he deserves his own deep dive. Blanda was a full-time starter for just one season (1953) before age 33, but the upstart American Football League offered him the chance at a second act to his career. All he did with it was throw for 20,984 yards and 188 touchdowns over his final 16 seasons in pro football — a period which, again, began at age 33. By age 40, Blanda was done as a regular QB, but he stuck around for another nine seasons as a kicker who occasionally spelled main starters Daryle Lamonica and Ken Stabler under center. I wonder how good Brady and Brees are at kicking field goals.

In many of these instances, the star quarterback had already been asked to take on less responsibility than in earlier days. Marino’s 218.6 yards per game in 1998 was one of the lowest marks of his career, and his Dolphins were running the ball more than they had in years. In a way, that shift mirrors a turn both the Patriots and Saints have already taken in recent seasons. New Orleans relied on the run in 2017 and 2018 more than in any other year this decade, while 2018 saw the Pats’ third-most carries in a season during the 2010s. In addition to their own unprecedented longevity, Brees’s and Brady’s teams have also put them in a position to succeed at an old age.

But the “gentle decline” option is not the one most productive old QBs take. In our sample of 29 QBs, 17 (or 59 percent) either fell off badly or had the good sense to retire — either way, that was it for their careers.

…And they fell off the cliff (or just retired) Among QBs with 10+ AV in a season at age 36 or older, those who failed to have at least one more 5+ AV season after their final 10-AV season, 1960-2018 Last 10+ AV Season AV in N Seasons After Quarterback Year Age AV 1 2 3 4 5 6 7 8 John Brodie 1971 36 12 4 3 — — — — — — Charley Johnson 1974 36 11 3 — — — — — — — Ed Brown 1964 36 10 0 — — — — — — — Chris Chandler 2001 36 10 3 1 1 — — — — — Steve Young 1998 37 19 2 — — — — — — — Rich Gannon 2002 37 18 3 2 — — — — — — Y.A. Tittle 1963 37 16 3 — — — — — — — Roger Staubach 1979 37 14 — — — — — — — — Johnny Unitas 1970 37 10 4 4 1 — — — — — Peyton Manning 2014 38 16 2 — — — — — — — John Elway 1998 38 14 — — — — — — — — Earl Morrall 1972 38 11 2 2 2 1 — — — — Phil Simms 1993 38 11 — — — — — — — — Fran Tarkenton 1978 38 11 — — — — — — — — Kurt Warner 2009 38 11 — — — — — — — — Craig Morton 1981 38 10 1 — — — — — — — Vinny Testaverde 2004 41 10 2 0 0 — — — — — Quarterbacks must be currently retired to qualify. Source: Pro-Football-Reference.com

The retirement option has its appealing case studies — chief among them being John Elway, who posted 14 AV for the 1998 Denver Broncos, won the Super Bowl and then moved on to his next careers as a business owner, team executive and Nerf football pitchman. (Brady could have done this last February, but decided to give it at least one more go — unsurprising, given his stated intention to play until age 45.) Fellow stars Roger Staubach, Phil Simms, Fran Tarkenton and Kurt Warner all retired after good seasons, too, perhaps seeing the writing on the wall despite still being productive.

Among the dozen who didn’t retire, however, the end was not pretty. We can break them down into a few categories:

Lost starting QB job: Chris Chandler; Charley Johnson; Ed Brown; Earl Morrall; Craig Morton.

Injured most of season: John Brodie; Steve Young; Rich Gannon.

Played very poorly: Y.A. Tittle; Johnny Unitas; Peyton Manning; Vinny Testaverde.

Some players could belong in multiple categories. Gannon, for instance, wasn’t playing well even before an injury ended his 2003 season, while Tittle played through an injury in 1964 and struggled as a result. But all involved a QB losing effectiveness (whether due to age or injury) and quickly reaching the end of his productive career.

Manning, Young and Gannon might be the most pertinent cautionary tales for today’s aging stars. Each was cruising along at an MVP level at an advanced age — exactly like Brees and Brady have done the past few years — but within a season of that, they were basically done as productive quarterbacks. Injuries played a big role in each case: Young had suffered multiple concussions by Week 3 of the 1999 season, Gannon was injured early in both the 2003 and 2004 seasons, and Manning missed Weeks 10-15 of the 2015 season with plantar fasciitis. If Brady and Brees do meet an unexpectedly swift end to their careers, it will probably follow the same pattern.

For now, of course, we don’t know how and when the end will come for the sport’s most prominent old-timers. Brady and Brees have already blown up the aging curve for quarterbacks, ushering in what The Ringer’s Kevin Clark called “the era of the Forever QB.” Brady just signed a new two-year contract extension, based off of the deal Brees signed with New Orleans in March 2018. In each case, both team and player are delicately balancing between committing to a franchise icon and keeping options open for the future. That’s probably smart: If the history of old NFL quarterbacks still has meaning, the end for Brees and Brady could come gently — but more likely, it will come without warning.