Matt Forte faces the daunting task of convincing teams running backs can produce after 30

The Chicago Bears are parting ways with Matt Forte after eight solid seasons and an average of more than four yards per carry in 2015. The biggest reason that the Bears wouldn't want to bring back such a durable and consistent running back appears to be the fact that he had a 30th birthday in December.

Perhaps no position in football drops off a cliff quicker than running back. The age of 30 at the position is older than 30 at any other. Marshawn Lynch of the Seattle Seahawks just elected to retire at 29 even if he might still have some left in the tank, but longtime Jacksonville Jaguars running back Maurice Jones-Drew looked done when he played his last game in the NFL before turning 30.

While the Pro Bowl rosters in 2015 featured 15 players at 33 or older, none of those were running backs and production after 32 is tremendously rare.

The 1,347 rushing yards gained by John Riggins at age 34 in 1983 are the most ever for a running back over 31, but it ranks as the 165th best rushing season of all-time. The 164 better seasons than Riggins' include just 10 seasons by 30- or 31-year-old running backs, and the other 154 were recorded by players 29 or younger.

All that spells trouble for Forte or any other running back trying to get a sizable paycheck this offseason after turning 30, which includes Reggie Bush, Chris Johnson and possibly some cap casualties like Arian Foster and Darren Sproles.

It's not a case of declining production for Forte, but a question of when that decline is going to hit. Frank Gore was one of the few running backs to actually get a decent contract after 30 when he signed a three-year, $12 million deal with the Indianapolis Colts, but he averaged 3.7 yards per carry in 2015 after tallying at least 4.1 in his 10 seasons with the San Francisco 49ers.

Still, the Arizona Cardinals and Pittsburgh Steelers each got steals by giving Chris Johnson and DeAngelo Williams $2 million deals last year and receiving solid 2015 seasons in return.

With veteran minimums bumped up in the new collective bargaining agreement in 2011 and reduced rookie contracts, there's more incentive than ever for teams to dump aging players and replace them with younger running backs.

That means Forte's next contract will likely look something like the ones given to Johnson and Williams, and nothing like the four-year, $30.4 million deal he just played out in Chicago.

Only Walter Payton racked up more rushing yardage in a Bears uniform than Forte, who tallied 8,602 yards in eight seasons, but history suggests his best days are in the rear view mirror, even if he outperformed his rookie backup, Jeremy Langford, in 2015. The Bears appear willing to run with the younger, cheaper guy even if his résumé doesn't stack up to Forte's.