Many quarterbacks have said that they want to play until they are 40. Drew Brees and Peyton Manning both said last year that they plan to do so.
Joe Flacco said last week that he wants to play until he's 40. But playing until he's 40 is a difficult thing to do.
Most quarterbacks that make it to the age of 40 are back-ups. Notable examples being Vinnie Testaverde, Warren Moon at the end of his Hall of Fame career, Doug Flutie, and Mark Brunell.
But it's not impossible for a 40 year old quarterback to thrive.
Flacco also has factors that are working in his favor. Like Favre, Flacco is one of the most durable quarterbacks in the NFL, as only Philip Rivers and Eli Manning have played more successive games than him, and he hasn't suffered any major injuries along the way.
Success is also a major factor in determining Joe Flacco's longevity. Fortunately for Flacco, success isn't an issue, as the Ravens have won 82 games during the Joe Flacco era and they show no sign of going into decline any time soon.
The big question that gets asked is whether Joe Flacco can make it in the NFL after his arm strength declines. But seeing as he's been successful in a West Coast Offense that focuses more on short and intermediate throws than long one, I'd say that Joe Flacco can be successful, even without having a massive arm cannon.
Joe Flacco could play until he's 40, if all the right things happen.