To be real, as far as I know, we have no real basis on how devil fruits work with stamina. For all we know, 70-year-old Akainu using his fruit would drain him just as quickly as Rayleigh tapping into Prime Strength or Haki. Or Whitebeard, in his prime, could pull the equivalent of a gatling earthquake barage, while in his old age he could only do a pistol earthquake.