Cartridge life really only 3 years??

I basically agree with Elizabeth.The suspension could deteriorate.Its sort of a rubber like material.Usually if it was exposed exposed to extreme heat,or something else to cause damage,then that would be different.This could happen to a brand new one also.But basically I have not heard of a shelf life standard time set for them.I imagine there are exceptions,like a company that tried a new material that didn't last as long as others.If it performs normal,I would use it without worrying.There are 30 plus year old ones, selling for high dollars yet,giving their new owners great satisfaction.

It is at least theoretically possible for rubber/elastomer used in the suspension of the cartridge to degrade with age, either from exposure to pollutants in the air (ozone), or because some of the compounds may be slightly volatile. This is the theory, though three years seems exceptionally short a shelf-life.

But, I haven't experience this myself, and I have heard many very old cartridges that sounded very good -- at least this is the case with the better brands.

If it ain't broke - replace it right away; because it might break at any moment.

The body and suspension will be fine, the stylus is another matter. Different cartridges seem to have different life expectency, some quote 1000, up to 3000 hours, before the stylus is degraded enough, to damage your precious vinyl.
Then do'nt throw it away, get it retipped. there are a number of excellent companies providing retipping services. Many cartridges can be retipped more expensively by the manufacturer. Some reasonably cheaply, such as Benz Micro, some at near the cost of a new cartridge, such as koetsu, but with Koetsu, it is, effectively, a new cartridge

OUCH!!! Did he mention that you should buy them from the same dealer by any chance?? Looks like he's realy 'hungry' and perswaysive to pitch you a new cartridge instead of the one you have.