Just digging into the age thing a little deeper - oldest QBs to win a Super Bowl:
Brady (43)
Brady (41)
Brady (39)
P. Manning (39)
Elway (38)
Earl Morrall (38)
Unitas (37)
4 years older than the next non-Brady. Insanity.
I feel like the blind squirrel who found a nut. 
I thought it might be fun to look into this claim. Looking at completion percentage, here are his last five seasons (regular season only):
2020: 65.7%
2019: 60.8%
2018: 65.8%
2017: 66.3%
2016: 67.4%
Numbers never tell the whole story, but other than 2019, I am not seeing any particular drop-off there.
Looking at deep balls, I found this data project at Football Outsiders. It’s interesting for stats geeks (which I am not, really). Here’s the takeaway numbers that I can find from that site regarding Brady and deep ball accuracy (defined as 21+ yards in the air) over the last 4 years, NOT including 2020.
2019: 46% (16th in league, slightly under league average of 47.7%)
2018: 37.5% (30th in league, well under the league average of 46.6%)
(methodology change)
2017: 47.1% (22nd in league)
2016: 44.7% (18th in league)
Those numbers are obviously not great, so I think it’s pretty fair to say that Brady is not a very good deep ball passer. But he’s fairly consistently not good, I would say, and 2019 was his best year of the last 4. So not really seeing a decline there, either. I’m not sure how the last few years compare to earlier than 2016, though, so maybe he was much better earlier in his career.
My recollection (which may well be hazy) is that Brady never had a cannon for an arm, but, as others have already noted, even as a younger QB, he made up for that with accuracy and intelligence. With the Patriots, he did an awful lot of short passing, and made slot receivers like Wes Welker and Julian Edelman into highly productive players.
Dear New England Patriots,
Don’t worry, chin up. So you let a franchise icon walk - it happens.
This, too, shall pass…eventually.
Sincerely,
1918 Boston Red Sox
Ah so you are saying that 2020 will be the first year of the Tommy Curse?
These numbers fit with observation. Only comment is that in 2016 Tom was 39 years old. So the decline on deep balls started before that. Back in those Randy Moss years his deep ball was unstoppable. He’s been in a different phase of his career for 6-7 years now.
QBs are like baseball pitchers in that there is a certain point where aging will take away your athletic abilities, but the really great players can adjust their play, and use their experience and skill developed over the years to compensate for that lack of athleticism, and even improve. Not every position in every sport has that luxury; you don’t see a running back or defensive lineman in football get better because he can mentally compensate for his physical decline. But QBs and pitchers certainly can. Possibly placekickers too (that I’m not so sure about).
In a few cases, this is likely the case. A guy who is generally a very accurate kicker, and who has been through enough pressure situations that he doesn’t get rattled by a kick with the game on the line, will likely still be valued even when his leg strength has started to decline.
Kickers like Jan Stenerud, Morten Andersen, and Adam Vinatieri kept kicking well into their 40s, even though, due to lessened leg strength, their teams may have had a second kicker on the roster for long field goal attempts and kickoffs (or had their punter handle kickoffs).
I knew about Vinatieri, in fact that is what prompted me to mention placekickers. But I wasn’t sure if he was an outlier or if that was something that any kicker could potentially accomplish. I’m glad there are other examples. 
If we subtract the end of Super Bowl 54 (which isn’t fair, of course), everything from the big 3rd and 15 play to the end, Mahomes’ overall Super Bowl stats are…not good.
In the other 112:45 of football, he went 45 of 81 for 451 yards, 0 TDs and 4 INTs. The really troubling thing is that he was fairly consistent in both games, other than the awesome streak at the end of SB54.
SB54 (the first 52:45): 19-32 (5.66 yards per attempt), 181 yards, 0 TDs, 2 INTS.
SB55: 26-49 (5.51 ypa), 270 yards, 0 TDs, 2 INTS.
He had major excuses in SB55, given the awful, patchwork offensive line. And so many of the plays were due to poor receiver play. But still, this is troubling.
Both those games were against two of the best front 7 defenses of the last 20 years. I’ll give him a pass.
Me too. I think he’s proven himself pretty well in almost every other game he’s played in the NFL. He doesn’t have very many bad days. I’d wager that every other team in the NFL wouldn’t mind having a QB as “troubling” as Mahomes. 
QBs are also like pitchers in that they hold the ball more than any other single player. Their decisions on what they do with the ball consequently has a tremendous impact on the game’s result.
That’s another great point, they are in the best position to be on-field “managers”.
Incidentally, this is why guards (usually point guards) tend to be important in basketball. The guy bringing the ball up the court is typically the one who decides where the ball goes, at least in the initial offensive set up. Give me a guard who can defend and rebound and then you’ve got a really valuable asset.
Of course that was the criticism for a decade or more about Peyton Manning of the Colts. “Awesome regular season QB; shame about the all-but guaranteed clutch failures in the playoffs.”
Manning eventually defeated the goat, or whatever it was. And eventually won a ring, got into the HoF, generally regarded as one of the best of all time, and all the rest of the honors.
But first he had to dig that hole then climb back out of it.
Mahomes, an amazing natural talent, may have a similar career arc.
You need some semblance of an offensive line to show off your QB skills. I’m not sure the Chiefs ever had that.
This really doesn’t describe Mahomes thus far, does it? Three seasons, 6-2 playoff record with one SB victory. One loss in OT in a conference championship game, the other loss in the SB.
Well he’s already won a superbowl and played in another, so no it won’t happen.