4. Rich Gannon — Retired at 39 in 2004
In fairness to Rich Gannon, he had a successful NFL career that saw him last in the league until he was almost 40 years old. Of course, Gannon was considered to be a journeyman quarterback until he was almost 33 years old, with teams being tantalized with this dual-threat ability to run and throw the football, but never being able to make the most of it.
But after joining Jon Gruden and the Oakland Raiders in 1999, Gannon became a Pro Bowl quarterback, and eventually helped the Raiders advance to the Super Bowl in 2002. But Gannon endured injuries to his shoulder in 2003 and his neck in 2004, which officially brought about the end of his career.