Peyton Manning will be 36 if and when he resumes his career with a still-unknown team.
With Mike Greenberg steering the Manning conversation this way, I visited Pro Football Reference for precedent. A search for the most productive seasons from quarterbacks at least 36 years old turned up memorable ones from NFC West alums Steve Young, Kurt Warner and Warren Moon.
The chart shows quarterbacks meeting that age criteria. Each passed for at least 25 touchdowns in a season. I've sorted them by NFL passer rating.
Manning is in another category while recovering from neck problems that sidelined him for the 2011 season. He also would be switching teams for the purposes of this discussion. Brett Favre (2009) and Warren Moon (1997) appear in the chart for their work in debut seasons with new franchises.
None of the players listed was coming off a career-threatening neck injury, however. Rich Gannon's career ended following one.