The 49ers' running back of the future is waiting in the wings in rookie Marcus Lattimore, but there's certainly no guarantee that future will be in 2014. Meanwhile, two younger, quicker backs on this year's roster – Kendall Hunter and LaMichael James – rarely get enough game action to get tired.The reason? Old Man Frank Gore seems to be anything but old.Gore, who celebrated his 30th birthday in May, is having one of the best seasons of his nine-year NFL career at an age when most backs start heading downhill.NFL history is filled with tales of elite running backs suddenly becoming shadows of themselves at 30, as time, thousands of hits and injuries take their toll.As ESPN.com NFL analyst Tristan H. Cockroft wrote a few years ago, "I've run the numbers and I've rarely seen a theory in sports that has stronger statistical evidence than this one: NFL running backs hit a wall once they turn 30. Nay, they hit a concrete wall, and it practically stops them flat."Yet Gore, at the halfway point of this 2013 season, is the outlier.As the 49ers (6-2) catch a break at midseason with their bye week – before resuming their schedule Nov. 10 against the Carolina Panthers at...