re-shelf life. Most rubber compounds react with oxygen from the atmosphere and lose their elasticity or structural integrity (they go brittle/crumbly)
There are, commonly available, many standard -size O-rings, made of different compounds for different applications. but you all know that!
An O-ring in an airtight , evacuated package, will keep indefinitely and should give a normal service-life.
No doubt there are special manufacture ones for aircraft use, specially compounded for contact with the fluids used in aviation. an honest manufacturer of integrity would make it clear that this was a "special" whereas one driven by greedy bean counters would insist that only"genuine" parts are used. (even though many components would be standard stock items.)
but once fitted they can go on indefinitely if not removed!
not so! At a molecular level, the O-ring can bond to the surface it's in contact with...disturbing the assembly tears the surface and destroys the seal. also degradation , as explained above, can occur.
Most mechanical assemblies need dismantling long before their seals and gaskets reach the end of their service-life. so you see very few perished seals.