Do MC Cartridges Deteriorate over time

I was gifted a high quality Clearaudio MC cartridge, that is probably 15 years old, but was used very little. It works fine, but wondering if it has sonically deteriorated over time...thanks
It depends on the hours of playtime, stylus must be retipped after 500-1000 hrs. If the damper is fine (you can check it) then you don’t have to worry about it. Most of my favorite MC cartridges are 35 years old, some of them still unused or even factory sealed, some of them are in use and sound quality is mind blowing compared to more expensive brand new cartridges.
Yeah, huge amount of high end audio sounds great in theory only to completely fall apart in practice. Like cartridges, the suspension should just get all hard and brittle and go to hell just sitting there used or not. But this never seems to happen. The Stanton 681EEE on my Technics from 1976 still works great, and that is a lot older and a lot cheaper than your Clearaudio. 

A good cartridge is like a good watch. Sure they will tell you it needs regular service. But in reality, If it ain't broke....
Suspension damping can change its characteristics overtime. With clean records I have seen styli last over 3000 hours. The only failures I have seen were due to failure of the coil wires at the solder joints. Copper will work harden so vibration at this joint eventually causes failure, This is the one big advantage of gold coils. They will never do this. I had a Sumiko Cartridge fail this way, a Talisman S. If it plays, tracks and sounds good to you it is probably OK.
If it plays, tracks and sounds good to you it is probably OK.

My thirty-five yrs old Highphonic MC-R5 outperforms certain modern top cartridges, tracks all the Telarc digital cannons with ease and the sound is just sublime, at 1.1 g VTF. Hard to beat for a modern MC or MM I´d say.
In theory, at least, the elastomers in the suspension can deteriorate over time, particularly if exposed to ozone and other such pollutants.  But, I've heard plenty of cartridges in the thirty year old range that sound fine and have not suffered from such problems. 

Way back in time, I had Shure cartridges that did not seem to last that long even though I was careful with setup and kept my records very clean.  When inspected, the report I got was that the stylus looked almost new, so what I was hearing was either the suspension going bad or stylus wear that was not that easy to see.  Whatever the cause, I have had MUCH better luck with wear with my MC cartridges (good thing, given the MUCH higher prices).

Aside from accidental damage (by far the biggest cause of cartridge failure), most of the better MC cartridges seem to last a very long time.  Most of the non-accidental premature death that I've seen had to do with internal wiring failure (evidenced by one channel going stone dead).
The good news for you is that you have no way of knowing how it might have sounded 15 years ago.  So, if it sounds good to you now, there is no use in worrying.  And as others have implied, there are expectations that cartridges deteriorate even in storage, but the sound quality one can often derive from even vintage cartridges calls that axiom into question.  
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I own several cartridges mainly MC some with 40 years of life (Nakamichi MC 1000 / Ortofon MC30) and have no problems, as explained to me a cartridge repairer is sufficient when they are not used to close them in their own packaging and not to let light take over.
But there are more sensitive mc cartridges than others about the dumper that could deteriorate; a dumper that is very durable and lasts for decades is the Monster Cable Genesis practically (almost) eternal while for the dumper of the cartridges mm they are very perishable worsening the tracing and the sound quality.