To what level do phono cartridges age?

Thanks for the info guys!

I have a 1975 vintage Audio Technica AT 15/20ss which I purchased new.  I have several NOS AT 20ss stylus' in stock and one AT 15ss stylus.  I recently swapped out a not too old stylus for a new AT 20ss stylus and I cannot say that I have heard any differences in performance between any of them other than they all sound far better than the original OEM one I still have.  But that original one has a lot of abusive hours on it, especially since it went through college with me.  What I cannot believe is the condition of all my original records which have been played hundreds or thousands of times through who knows how many wild parties!   I pulled out some old records I have had since the 60's/70's and 80s and they are still pristine :) 

Actually moving magnet and iron cartridges should last as long as their styluses regardless of age. The problem with moving coil cartridges is that copper work hardens. The vibrations travel down the fine wire to the point were they are soldered to a terminal which serves as the fulcrum. The wire will eventually break here. Gold does not work harden so theoretically cartridges with gold coils could last a very long time. I have had two cartridges die this way. It does depend on usage.
Some people believe the suspension changes or hardens with age. I do not know if this is the case. I can't tell that it is as the change would be so slow. I have not noticed this in any of my cartridges. If it is true the compliance of the cartridge would drop over time and your resonance frequency would rise.