29-year-old Steven Jackson has struggled to his worst season as a pro. 29-year-old Frank Gore is averaging 5.5 yards per carry.
The St. Louis Rams and the San Francisco 49ers are both—at least ostensibly—paced on offense by 29-year-old running backs who broke out in their sophomore seasons and came into the league as above-average receiving options. Since his 2300-yard 2006 season Steven Jackson has been one of football's most dependable running backs, crossing 1000 yards each year. Since his 2000-yard 2006 season, Frank Gore has struggled to remain healthy but constantly flashed big-play ability.
Now they're both healthy—at least as healthy as 29-year-olds can be—but the status quo has definitely shifted. As Gore looks on pace for his best season since that peak year, averaging 5.5 yards per carry, Jackson has receded into the background in St. Louis, caught up in trade rumors that didn't pan out and struggling to maintain his touches against a seventh-round draft pick.
Gore's not running what looks like a full-time workload, either; he's only got 11 more carries than Jackson through eight games. But he's only carried the ball 300 times once in his career, so we're used to it, by now, while Jackson's identity is in question now that he's not a stereotypical workhorse.
There's still a half-season left for things to change, and the way sports work, they probably will. But for now, the Rams and 49ers' 29-year-old running backs are on vastly different trajectories, for the first time in their careers.