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...There were only a handful of quarterbacks that made it to past 36 and even fewer who were 40+ year old NFL QBs and all of them played for multiple teams.
Good comment. You got me thinking. Let's look at some facts.
There are 17 SB-era, HOF QB'S: Aikman, Bradshaw, Dawson, Elway, Fouts, B. Griese, Jurgensen, Kelly, Marino, Montana, Moon, Namath, Starr, Staubach, Tarkenton, Unitas and Young.
Their average retirement age was 37.4; the median was 38.0. The oldest was Moon at 42. Aikman, Bradshaw and Namath were the youngest, retiring at 34 (nice work if you can get it). For reference, Montana and Elway were 38 in their last season.
In their final seasons, they started an average of 8.9 games and a median of 10.0 games. Their average and median W/L percentages were .528 and .517. Their average Pass Rating was 73.1, nearly identical to the median of 73.2.
In their next to final seasons, they started an average of 11.6 games and a median of 12.0 games. Their average and median W/L percentages were .559 and .583. Their average Pass Rating was 76.5, the median was 80.0.
The average age at which they made the Pro Bowl for the last time was 34.5 (median 34.0). Moon was the oldest to make the Pro Bowl, at 41; Elway made it at 38. Aikman, Bradshaw and Starr made their last Pro Bowls at the ages of 30, 31 and 32 respectively. Montana made his last Pro Bowl at 37, as did Staubach and Young.
The star of this group by these measures was Elway who started 12 games in his final season with a W/L record of 10--2 and a Pass Rating of 93. Montana started 14 games with a 9--5 record and a Pass Rating of 83.6 in his final year. Namath, Starr and Unitas had Pass Ratings of 54.5, 45.2 and 40.0 respectively in their final seasons, but with very few games started. There is a wide range among the others in terms of Games Started and Pass Rating.
In general, these QB's stuck with one team for all or most of their careers.
Aikman, Bradshaw, Elway, Fouts, B. Griese, Kelly, Marino, Starr and Staubach played their entire careers with one team.
Dawson spent virtually his entire career with KC, except for a few games in his first two years. Montana spent the last two years of a 15 year career with KC, including a trip to the AFCCG and Pro Bowl. Namath spent his entire career with the Jets, starting four games for the Rams in his final season. Unitas started four games in his final season with SDG after 17 years with the Colts. Steve Young spent 13 years with the Niners after two seasons in Tampa Bay.
The exceptions to the longevity with one team rule were Jurgensen who split his career between Philadelphia (seven seasons) and Washington (11 seasons); Moon who spent 10 years with Houston, three with MIN, two with Seattle and two with KC (where he only started two games) and Tarkenton who spent the middle five years of an 18 year career with the Giants and the rest, of course, in MIN.
Bottom Line: HOF QB's generally play for one team and tend to tail off after their mid thirties, with, of course, notable exceptions like Elway and, to some extent, Montana.