NFL Aging Curves by Position: Rookie QBs, Third-Year WRs, and Age-30 RBs

(Photo by Jed Jacobsohn/Getty Images)

They say age ain’t nothing but a number.

Twenty-one, 26, 31—what’s the significance? Is age merely just a number, or a baseline for seasonal performance? How can we quantify this?

Aging curves—also called aging patterns or age factors—show the relative performance of a group of players for each age, usually either showing how much (in percentage terms) a statistic improves or declines from one age to the next, or how the production at any age compares to the peak age.

One method used for aging curves it to simply add up the stats for every player at each age and look at the resulting sums. This is flawed, however, for it doesn’t account for the fact that there are many more players at age 24 than at age 34—the difference in yards or touchdowns per attempt won’t offset the disparity in attempts.

You can solve this problem by dividing the sum by the number of players at that age, right? Well, technically, yes, but it’s still not enough.

There’s selective sampling issues using this technique: The players who rack up attempts or receptions at age 21 or 22—their rookie season—are usually the best players, and thus it would appear that a player’s first year or two is one of their best.

At 35 and 36, the only guys left are the ones who have had a very successful first 12 years of their career and, as a whole, are typically better than the average player at age 24 or 25...and thus, it would appear that a player also peaks very late in his career.

The solution is to look only at matched pairs: how each player performs from one year to the next. Instead of comparing the average production at age 24 to age 25, you see how those 24-year-olds do the very next year. This way, you are looking at the same players in back-to-back years; previously, not every player was the same in each sample.

So, if those set of 24-year-olds have a completion percentage of 60 percent, and at age 25 that rises to 63 percent, we are fairly certain that completion percentage is 1.05 times higher (63 divided by 60) at age 25 than at age 24.

But we’re forgetting the most important principle at hand: regression to the mean. Observed production is equal to a player’s true talent, plus luck or random noise. What we are trying to do is to eliminate that noise, because, in general, that luck goes away in the next year. (For instance, running backs with over 1,000 yards since 1980 average 6.23 yards per game less the following year, almost 100 yards in a full season.)

We want to regress a player’s Year X stats, but leave the Year X+1 stats alone. The question is, how much should we regress? I found that a quarterback with 512 attempts, a running back with 439 attempts, and a wide receiver with 45 receptions should have their yards per attempt or reception regressed 50 percent to the mean; there are different rates for completion percentage and the like, and the more attempts or catches a player has, the less he’ll be regressed.

Normalize each player’s two years based on the league average, and we’re finished.

Let’s see how these aging curves stack up for each position.

(Note: My data sample ran from 1980 to 2007. I looked at quarterbacks with 50 attempts, running backs with 30 attempts, and wide receivers with 20 receptions in Year X, no matter how many they had in the next year, because I weighed each of the players’ two years by the minimum number of attempts or catches in the back-to-back years. I excluded any players who switched teams mid-year or in the offseason.)

(Another note: Unless otherwise noted, aging curves shown are "chained" and then divided by the peak level. That is, if yards per attempt falls two percent every year from age 21 to age 30, then age 21 would be given a value of 1.00, age 22 would be 0.98, age 23 would be 0.98 x 0.98, or about 0.96, and so on until 0.834 at age 30. The numbers are then divided by the peak value, 1.00 at age 21. In other words, a player’s yards per attempt would be 83.4 percent of what it is at age 21.)

Quarterbacks

Some might say that a quarterback’s best years are in his mid-20s, pointing to common knowledge and the successes of Daunte Culpepper and even Marc Bulger. Others may bring up Tom Brady, Peyton Manning, Kurt Warner, and Brett Favre as guys who succeeded well into their 30s.

Who’s right?

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