Here are 4 Rb's who played at age 36 and their stats:
Marcus Allen (Chiefs), 1996 & 1997: 1,330 rush yards, 4.0 yards per carry, 20 TD
John Riggins (Redskins), 1985: 677 rush yards, 3.8 yards per carry, 8 TD
John Henry Johnson (Steelers/Texans), 1966 & 1967: 237 rush yards, 3.2 yards per carry, 3 TD
Joe Perry (49ers), 1963: 98 rush yards, 4.1 yards per carry, 0 TD