It's a much more difficult question than it first seems. Averaging the performance of all the recent QBs by age doesn't work. A survivor bias ensures that only the successful QBs stay in the league long enough to have their stats in the sample. Another complication is the the steady inflation of passing stats over the years. In this post, I'll try to tackle those problems to better understand how QB performance is affected by age.
For this analysis, I used Adjusted Net Yards Per Attempt. (Other stats such as EPA or WPA don't go back far in enough in history for a usable sample size.) Adjusted Net YPA starts with simple yards per attempt numbers. 'Net' means that sack yardage is included and sacks go in the denominator as pass plays. 'Adjusted' means that a 45-yard penalty is included for each interception. And because the data came from Pro-Football-Reference.com, a bonus of 10 yards is included for each TD pass. I only included seasons where the QB had at least 10 starts in the regular season, and I only included QBs who were drafted in 1978 or later, which is considered the modern passing era in the NFL.
First, let's address the inflation of passing stats over the years. The graph below plots the steady increase in all QB AYPA over the years. This trend has the effect of masking a QB's true decline. A hypothetical QB who throws for 5.0 AYPA every year of his career is actually declining with respect to the rest of the league. To correct for this trend, just like economists adjust for inflation, I chose a reference year and added or subtracted the appropriate amount for each QB's stats in each year. In this case, I chose the approximate mid-point of the period, 1990.
The problem of survivor bias is tougher. Each QB will have his own unique career pattern. Some are quick studies, who learn from their early experience, while others are slow on the take. Some rely on their foot-speed, which declines more with age than other physical attributes, while other QBs rely on their accuracy and decision-making. Some QBs seem to have supernatural, age-defying abilities to heal, while other QBs have bodies that break down quickly. Survivor bias means that we'll only see QBs who perform well enough to remain in the league.
So if we simply average AYPA by year, we get the following graph:
It appears that QBs don't decline with age at all! In fact, they just keep getting better and better! We know this is survivor bias at work, so we need a different way to look at age and performance.
One way to begin is to fit the analysis to the specific question asked. In this case, I wonder what kind of decline can we expect from Peyton Manning as he continues to age? He's had some incredible seasons, and how will his remaining seasons compare to those? So I looked at QBs like Manning, who already have proven themselves by starting several years, specifically 10 or more seasons. I averaged each QB's decline from his peak years, defined as the average of his three top seasons, at each age. For example, Chris Chandler peaked at 7.0 AYPA at age 27, 30 and 31. At age 32 he put up 5.5 AYPA, which is a decline from his peak of 1.5 AYPA. Averaging franchise QBs' improvements and declines with respect to their own peak years helps overcome the selection bias, and produces the following aging pattern.
Here we see a pattern consistent with what we'd expect. Young QBs gain from experience and possibly from physical development, until a peak age at which the effects of age overcome the added experience. For established 10+ year starters, the peak is at about age 29, and the decline appears relatively shallow, with performance dropping about 0.5 AYPA over a 7 or 8 year period.
A similar approach is called the delta method. This method looks at consecutive year-pairs, and it measures the improvement and decline from one season to the next. By averaging all the QBs' change in performance by age, we can see the effect of age without the survivor bias. For established 10+ year starters ("franchise QBs"), the results are fairly linear:
The graph above can be read like this: Values above zero indicate improvement, and values below zero indicate decline. For example, a QB aging from 21 to 22 will tend to improve by 1.0 AYPA. By age 30, the decline has begun, consistent with the previous 'peak difference' method. Another way to look at the same data is as a cumulative improvement/decline by age, as shown below.
In the above graph we see rapid improvement until a plateau beginning around age 26, and then a shallow decline beginning at age 29.
So far we've only looked at QBs who have 10 or more years as an established starter, something we already know about Manning. Let's open the aperture and look at QBs with 5 or more years as an established starter. I chose 5 years because I'm interested in real-world QB decline, and not just theoretical physiological effects of age. Guys with fewer than 5 years probably aren't good enough to survive past their peak.
The delta method for 5+ year starters produces the following aging pattern:
The aging pattern for 5+ year starters looks very different than for the 10+ year guys. The peak is not as high and the decline goes further. The magnitude of the improving years for the 5+ years group does not peak as high as for the 10+ years group, which makes the decline appear stronger. If we took a QB who has started for 5 years and no longer, and knew nothing else about him, this might be the aging curve we'd expect.
One of the lessons here is that there is no one true aging pattern for the league as a whole. It depends on the type of guy and the comparison data used in the analysis. For example, if you perform the delta method for all QBs, regardless of career length, you see a flat plateau from age 22 through 28, with almost no improvement. This is because the the many young QBs who never catch on weigh down the averages for everyone., much like we saw in the first graph of simple averages by age, which was one big plateau.
One of the more interesting things in the numbers was that the final year of a QB's career, regardless of age, is usually pretty bad, but not necessarily worse than the usual year-to-year variation in any individual QB's resume. In fact, the final year of a QB's career, on average, represents a decline of -0.75 AYPA. This is far worse than any one year of average decline due to age--actually equivalent to about 6 years of decline. To me, this suggests that natural variance is helping end many QB careers.
In fact, if you simply remove the very last season of each QB's career from the data, age-related decline virtually disappears. Here is what the aging pattern would look like:
A lot goes into the decision to retire, and it's not always completely the player's choice. Older QBs are checking out of the league after a down year, but there's no guarantee that the downward trend would continue. Although it's unlikely they'll reach the highs of their peak years, regression to the mean says that the following season is more likely going to be an up-year, at least relative to the previous one. At some point, a QB has absorbed enough sacks, had enough surgeries, made enough money, won enough thrillers, and lost enough heart-breakers for a lifetime. If the prospects for future success aren't very good, it's time to hang up the cleats, even if those prospects are somewhat of a statistical illusion.
The bottom line is that very successful quarterbacks like Manning aren't going to become bad slowly. All of sudden one year, they'll have significant drop-off in performance. If they were 26 and had the same kind of season or had a similar injury, they'd no doubt be back at camp the following July. But at 36, that job in the broadcast booth will seem quite enticing. Successful, established QBs will generally continue to be successful until one day they're not. We won't see it coming. But of course, everyone will pretend they did.