Cartridge aging and tip wear symtoms?

If you can hear surface noise or any other effects of stylus wear, you've probably already done permanent damage to your vinyl.

Assuming clean vinyl and proper setup, the main factor in maximizing stylus life is the size and shape of the stylus. The larger the contact area, the less friction at any given point and the longer the stylus will last. Conical (spherical) styli have the smallest contact patch (theoretically a geometric point) and therefore the shortest expected life. Elliptical styli are better. Line contact stlyi are better yet and micro-ridge styli should last longest of all, since they have the largest contact surface. The makers of a few, high end micro-ridge styli even go the extent of aligning the ridges with the molecular structure of the diamond, which makes them significantly more resistant to friction.

Stereo shops used to have specialized binocular microscopes for stylus inspection. Even with that it takes a trained eye to know what to look for, and fewer shops have those tools and skills any more. We have one locally but many cities do not. You could consider mailing it to the manufacturer or one of the respected retippers, like Soundsmith for example. They'd give you an honest assessment.

An audio signal analyzer or software could probably be used for this also, assuming you had data from a new sample of the same cartidge to compare. A worn stylus would start to lose HF response, beginning with highest frequencies the cartridge can reproduce. The more wear, the more this would progress. You can't wait until you hear this however. Most cartridges extend well above 20KHz, so by the time you noticed HF rolloff much damage could have been done.

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.