It would be no surprise if 39-year-old Peyton Manning decides to retire from the NFL after a pro career that began in 1998. But the recent retirements of Seahawks running back Marshawn Lynch and Patriots linebacker Jerod Mayo at the age of 29 came as a bit of a shock. Reports that Lions receiver Calvin Johnson at age 30 is also considering stepping away raises a question: Are NFL players having shorter careers?

According to data from Pro-Football-Reference.com, NFL careers are shrinking at an unprecedented rate. From 2008 to 2014, the average NFL career dropped in length by about two and a half years.

The decrease in career lengths is a historical abnormality. From 1991 to about 2008, career lengths were mostly consistent. But since 2008, players have been exiting the league earlier.

While the decrease is occurring across all positions, some have seen a sharper drop. Quarterbacks have seen their career lengths fall by almost three years during this span, with the average career across all positions falling from 4.99 years to 2.66.

Concerns about the effects of concussions and head injuries have prompted some players to retire early. Several recent retirees, including Chris Borland and Adrian Coxson, expressed concerns about their long-term health when discussing their reasons for leaving the game.