"I purchased a pair of your glasses 4 or more years ago. Lately, I’ve noticed that my eyes are hurting when I am done at the end of the day. Does the filter material “wear out” over time?"
Answer: No. The filter elements are part of the glass matrix and cannot/do not fade. Welding filter glass uses iron as the major filtering element. Blue Didymium/ACE glass uses didymium/neodymium/praseodymium plus other “rare earth” elements. These elements are mixed in with the silica when the manufacturer is melting the glass.
The manufacturer of the glass routinely tests its glass over time, keeping samples of each melt and subjecting them to various tests to determine transmission over time and exposure. Schott Glass Technologies, for example, has samples of didymium that go back over 50 years, and that glass still has the same transmission characteristics that it had when it was first made.
What is more likely is that over time, your skills have improved from the time you first bought your glasses. You may be working longer hours, you may be using different techniques, and you are certainly working with different formulations of borosilicate glass (especially the colors). You may also be doing more fuming now than you did when you first got your glasses.
The bottom line is that you have perhaps “outgrown” your current filters and need something with more filtration.