These guys did an aging curve (based on pro-football-reference's approximate value statistic) for every offensive position.
Here's the chart for quarterbacks:
View attachment 1002
Approximate value is calculated at the end of the season, but using a back of the envelope calculation, it can be derived from adjusted net yards (ANYA times attempts): approximately 270 adjusted net yards per "approximate value" unit.
So far this season, Brady is on pace for ~5350 adjusted net yards. I don't think he gets there, because of the injuries to the receiving corps and the colder weather, but even if we knock off 500 yards from that total, we're looking at an approximate value estimation of 18 or 19.
If you peg everything to his peak (age thirty) season, which was -- you guessed it -- 2007, you conveniently get back a -5 or -6 for Brady's adjusted value versus peak. So it would appear that he's following the normal aging trajectory, except for the fact that 2007 was basically an outlier. Outside of 2007, Brady's performance has been remarkably consistent, as you can see in the chart below:
:
View attachment 1001
Who else has performed similarly to what Brady is doing in his late thirties? It's a short list: Manning, Elway, Favre. Let's stretch a little and add Flutie (!), Warner, Garcia, Simms, Montana, and Moon.
Now let's scratch off everyone who retired (or otherwise never played another down) after their age 38 season. That leaves us with Favre, Manning, Flutie, and Moon. In the seasons following their age 38 season, these guys had the following performances (by AV):
Favre (14 AV at 38): 12, 16, 5 AV
Manning (16 AV at 38): looking like 6-8 AV this year
Moon (11 AV at 38): 13, 4 (injured), 12 AV
Flutie (6 AV at 38, backup to Rob Johnson): 11, 0 (backup), 5 (backup) AV
Manning is the cautionary tale, here: a whopping 10-point drop in AV between his age 38 and 39 seasons. But guys like Favre, Moon, and to a lesser extent, Flutie, all seemed to maintain their level of play for another two seasons or so. So it's entirely possible that Brady might suffer catastrophic collapse in his level of play in the near future, but he doesn't really have Manning's red flags (neck injury). He should be able to sustain, with some decline, for another two seasons or so. On the other hand, if he wins another Super Bowl, he could abruptly retire, like Elway did.
Why all this long rigamarole? Because I'm trying to figure out how to rate older QBs like Brady in this exercise. The risk of total collapse is very real, but Brady's talent is extremely rare. At the end of the day:
Rodgers
Brady
Wilson
Newton
Brees
Luck
Dalton
Winston
Carr
Roethlisberger
Romo
Ryan
Rivers
Palmer
Cutler
E. Manning
.. after that, I'm not going to bother.
Maybe I'm overrating the old guys, but young players are risky, too. Andrew Luck is a great example of that.