During the 2020 season Frank Gore turned 37, making him the oldest running back to carry the ball in the NFL in many many years. Emmitt Smith only played until the age of 35. Adrian Peterson played until the age of 36 (although he only had 38 carries in his final season). Even some of the greatest running backs to ever play can’t outrun Father Time.
Unlike quarterbacks who can play until the early 40s if they are lucky or good (or both), running backs tend to lose their jobs when their skills fade even a tiny bit.