Back in March, Chase wrote a post investigating how quarterbacks age, finding that they peak at age 29 (with a generalized peak from 26-30) in terms of value over average. Today, I thought I’d quickly look at how quarterbacks age in terms of their performance rate — specifically, their Adjusted Net Yards per Attempt (ANY/A). For newer readers, ANY/A is based on the following formula: (Passing Yards + 20 * Passing TDs – 45 * INTs – Sack Yards Lost) / (Pass Attempts + Sacks).

First, I need to introduce a way of adjusting ANY/A for era: Relative ANY/A. Relative ANY/A is simply equal to:

QB_ANY/A – LgAvg_ANY/A

The table below lists the 30 single-season leaders in Relative ANY/A since the merger. You won’t be too surprised to see the 2004 version of Peyton Manning at the top. That year, Manning averaged 9.8 ANY/A, while the league average was just 5.6 ANY/A. That means Manning gets a Relative ANY/A grade of +4.1 (with the difference due to rounding).

Using this, we can evaluate every quarterback’s season independently of era, and compute the year-to-year differences in Relative ANY/A at every age.

Taking every quarterback who had at least 15.1 dropbacks per game (which tends to correspond to the standard 14 attempts per team game) in back to back seasons, I fed the year-to-year Relative ANY/A deltas into a cubic regression and smoothed out an aging curve. (This is the same process I used to calculate an aging curve for basketball players for ESPN Insider last year.)

According to this methodology, here’s how a QB can expect his Relative ANY/A to change from year to year at each age:

Or in graphic form:

This would indicate that on average, quarterbacks peaks at age 27. To put the data in another light, if we created a passer who peaked at 8.0 ANY/A, and he perfectly followed this age curve, here is how his ANY/A would look each season:

One big caveat with this study: there’s probably a good deal of selection bias here, in the sense that only passers deemed to be good enough to keep playing will get a chance to put up 15.1 dropbacks/game the following year. Still, I think this provides a pretty good estimate of how much improvement/decline you can expect from a guy at a given age.

Armed with this aging curve, we can do a lot of cool things in subsequent posts, so stay tuned….