Can logia fruit users die of old age? For example, portaguse.d ace. I'm asking this question because logia users are the element of the fruit they eat, and i'm pretty sure elements can't age.
No, if that was the case, Fire-fist Ace wouldn't have died in the first place. Logia users aren't always the element -- you can read it here -- and, yes, Logia eaters can die of old age.