Ray, Obviously, it's a case by case phenomenon and past history of storage conditions must be an important determinant of cartridge longevity, but there is enough collective experience on this thread alone to put the kibosh on your thesis that cartridges only last 18 months from manufacture, granted that you are only repeating what you learned from two reputable sources. In my 35 years of playing vinyl, I never ever noticed any sort of deterioration in a cartridge after such a short time. In fact, since I don't get that many hours per week to listen, many of my cartridges (purchased new from dealers, not NOS) were still getting better at the 18-month mark. Also, I used to haunt audio salons in the good old days; I never heard any dealer make such a blanket claim re shelf life of cartridges. (By the way, why is Laura Dearborn qualified to expound on this subject, other than the fact that she wrote the book you cite?)
