The Washington Post asked an interesting question: Are NFL careers getting shorter? When they ran the numbers, they found the answer was yes, and by a frankly stunning number of years.
To quote Deadspin, since the article is behind a paywall and you won't all want to pony up:
Using data from Pro Football Reference, the Wall Street Journal has run the numbers and found that as of 2014, the average NFL career is 2.66 years. As recently as 2008, that number was 4.99 years.
There isn't any one reason for this, as the article notes, but it's still pretty sobering that from the start of Mike Smith's tenure in Atlanta until the end, the average NFL player was playing two fewer years in the NFL. There have been early retirements, injuries aplenty, and as Deadspin notes, increasing specialization of roles. None of that fully explains such a huge drop, but together perhaps they do.
What is clear is that the NFL is fast becoming the kind of place where only elite players can make an extravagant living for themselves. If anything's going to really kill the seemingly invincible brand, it will be players turning away, so this is something to monitor in the years ahead.