Cartridge aging and tip wear symtoms?

I am quite happy with an original Fidelity Research Mk13F, into a Sutherland PHD on a Rega P9.
I bought the cartridge brand new back in the 1980s and I firmly believe it sounds better now than when new, but of course all the ancilliaries are better.
No noise issues or cantilever failures,is this a fluke or do they just not make them to last anymore?

distortion in the inner groves

There's no distortion in the inner grooves when playing the old FR cartridge.
An improperly set up cartridge that is brand new will have distortion in the inner grooves also.
Closer inspection using powerful magnifying glass or jewelers specs would show obvious stylus wear, especially on either the right or left side of the tip, indicating that the cartridge was mistracking for quite some time.
The cantiliver they say can loose it's stiffness over time , but so far after 20 years the FR is still strong.
Dropping the arm on the record not lowering, and tracking at too high a range can lead to cantilever fatigue in a brand new cartridge.
If everything is set up to spec and there have been no mis-haps,and still lp's just don't sound as good as they once did, time to invest in a new cartridge and make sure it is compatable to your arm and set up properly.
Most cartridges fail because somewhere along the way,an accident happened.

Frequently the suspension will give up before the diamond ever starts to show wear. Doug is right, unless you have the proper stereo microscope it's almost impossible to tell how the diamond is wearing. I used to do it, and what I would typically find is dirt! Back then, most people didn't practice proper stylus cleanliness, so you ended up with a pack of crud on the end of the cantelever. Once that was cleaned up, then next thing was the impact of improper alignment / anti-skating. Many stylii would show uneven wear. But a properly aligned, properly maintanined (including clean records) cart ridge could last a couple of thousand hours. Most people replaced carts because they wanted something new long before their original cartridge wore out.

I've viewed many needles under the scope over the years. Rarely did I see one that was actually worn. I did see a few where the stylus had been sheared off of the cantilever... and a lot of dirty styli :)

As the cartridge ages, usually whether it is played or not, the cantilever suspension will perish. As this happens, the cartridge will begin to mistrack; often it will sound harsher. With most cartridges 2-3 years is about all you can hope for before the cantilever has to be refurbished.