FWIW: I made two identical exposures today on 11x14 Polymax Fine Art FD. One from a box I bought on Tuesday, and one from a box that I have had for over a year. They were contact prints and look identical. I am "past dates" a little on the last box, and the former says, "Use before 4/05." I recently used a 14 year old sheet of Elite, Gr. II and it was toast.
Whatever "edge" my brand new PMFA has over the two-year old stuff, I cannot see. If stored in a darkroom whose temperature always stays between 55 and 75 F., I am sure that fiber paper will last several years before it goes bad.
I am putting the 14 year old Elite on E-bay as "pre-flashed." ;-)