The San Diego Chargers are the latest team to confront a decision on whether to keep a star running back who’s on the wrong side of 30. The Chiefs are waiting in the on-deck circle.

The Chargers decided to restructure the contract of LaDainian Tomlinson, who’ll turn 30 in June. Formerly the NFL’s dominant running back, his production has slipped in each of the past two years and he’s been hobbled by injuries for the last two postseasons.

The Chargers needed salary cap relief from the $24 million due Tomlinson through 2011, and the options for both were restructuring or divorce. Because Tomlinson still ran for 1,110 yards last year and remains the face of the franchise, the Chargers sought to keep him.

The Jacksonville Jaguars, however, released Fred Taylor, a seven-time 1,000-yard rusher who’s 33 and had become a backup. But Taylor’s age didn’t stop the New England Patriots from signing him. The Patriots have a knack for getting useful mileage from the over-the-hill generation and for them, Taylor could fill the right role at the right price.

Which brings us to Larry Johnson, the Chiefs’ two-time Pro Bowl runner who turns 30 in November. He hasn’t had a 1,000-yard season since staging a holdout in 2007 that resulted in a five-year contract extension. His future is among the decisions facing Chiefs general manager Scott Pioli and coach Todd Haley.

Their most logical approach would be to first evaluate Johnson’s current ability, then decide if he still has enough upside as a back to even bother sorting through his off-the-field baggage.

Evaluating Johnson’s past two seasons is a bit of a puzzle. Was the foot injury that knocked him out of the second half of 2007 proof that his NFL-record 416 carries in 2006 have permanently diminished his running skills and durability? Or was it just a routine football injury?

Was his up-and-down 2008 season, in which Johnson still averaged 4.5 yards per carry, an indication he’s no longer a consistent threat to move the chains? You could argue he had a credible season, considering that his blocking was shaky early in the season and then he was asked to adjust to a spread offense, which makes a power runner like Johnson a fish out of water.

Johnson’s age definitely doesn’t work in his favor. Thirty has become a witching hour for NFL running backs, including the most productive ones in Chiefs history.

Christian Okoye, the first Chief to win an NFL rushing title, was 26 when he was drafted in 1987, so you might have figured that he’d still have a lot of tread on his tires at 30. Yet the year Okoye turned 30, in 1991, he enjoyed his last 1,000-yard season. In 1992, he suffered knee problems that ended his career.

Priest Holmes, the best free-agent acquisition in Chiefs’ history, enjoyed a sensational run from 2001-2003 that, if sustained, would have made him a Hall of Fame candidate. But he turned 30 in 2003 and in each of his last three seasons he could not stay healthy for more than eight games.

Johnson isn’t as critical to the Chiefs’ future as he seemed to be in 2007. Though the Chiefs were rolling the dice by committing so much cash to a back who’d just run through a meat grinder, another big season from Johnson was crucial to their chances for a decent offense and another playoff run. But once Johnson was injured in the eighth game, the Chiefs did not win again.

Because his holdout stretched into mid-August, time was on Johnson’s side in 2007. But for a back pushing 30 and with a franchise moving in a new direction, that’s not the case any longer.