Is there any noticeable difference in sound quality if you use a new XLR cable instead of a well-used 10 years old one?
I work at our local church and can borrow a cable from there to do some recording for a short film instead of buying it..
If you use a new crappy cable, it's going to sound worse than a well-made and cared for high-quality cable that's 10 years old.
But by the same token, a well-used workhorse cable that's been cared for properly for 10 years might not sound as good as something studio-grade.
Age really has nothing to do with it. Quality, construction, and care are what matter here. When care is absent, then age makes a difference--but that would be visually noticeable.
Yes, they do wear out eventually. As pointed out already, the life time depends greatly on use and quality. Even with a cable that seemingly works sound quality may get drastically reduced. See below.
The type of defects you will start to see over time, can be categorized into three main areas:
Obviously this is the most likely thing to happen: the constant pushing and pulling in the connectors and cable will eventually cause failures and degraded signal.
On the contrary to what one might think, these problems often DO NOT show up as total failure at first hand. It may be one of the cobber strands from the shield that shorts to the hot or cold connector, or the cold/hot shorting. This will not take out the signal completely:
Depending on the type of short, this may up working as high pass filters (killing all low end), as a low pass filter (taking out the highs) or take out / degrade the phantom power capabilities. This is probably the most common failure (it is working, checks out fine on the tester, but sounds dull or causes troubles with phantom power).
The connectors them selves may be cracked allowing fluids and dirt to enter or start to break into pieces. This can cause cracks and pops etc.
The cure is to trim and resolder the cable ends.
Corrosion on the pins and holes in the connectors is also quite common with old cables. You can easily spot this as they loose all the shine, may change color and may be very hard to pull out.
Sometimes this is easy to clean with an anti-corrosion spray, sometimes not. Alot has happened with the quality of connectors the last 10 years. If you have really old, but good cables, a new set of connectors (along with the resoldering that comes with it) is the remedy.
With cables used out-door or in longer periods in high humidity, internal corrosion may also show up on the cobber strands them selves (due to moist). In particular if the plastic/rubber shield is broken or the wrapping is really poor/loose. The result is again degraded connectivity, leading to the same type of filtering/phantom power issues mentioned above.
The rubber shield start to loose its flexibility over time, get brittle and may even crack.
The obvious problem here is that it leads to corrosion and/or damage on the shield.
The less obvious problems are that when this also happens to the core insulation, which raises the capacitance and lowers the resistance of the insulation.
It may also loosen up the insulation/layering and the cable could then become microphonic (yes, stepping on the cable, or letting it fall towards the ground will actually produce a sound). Even if the outer shield seems all good, but the internals are loose, it may case moist to get absorbed, which again leads to corrosion.
There is no cure for the actual cable here (you can maybe salvage the connectors though).
In addition to Michel Hansen Buur's answer, here are some common causes of cable degradation:
people stepping on the cable (and other point loads). Every time you do this, the cable deforms a bit, and the wire strands rub against each other. Over time, individual strands will break. Do it often enough, and one of the wires will be severed completely, or worse, you get an intermittent fault.
coiling the cable too tightly. Again, this puts mechanical strain on the cable. Cable manufacturers specify a minimum bending radius. Rolling up a cable by coiling it tightly over your hand and elbow is one way to violate the min. bending radius. Tying knots into a cable, and looping the loose end of a cable around all the windings in a coil also result in too-tight cable radius.
coiling the cable while not undoing the twist that you introduce. The twist introduces mechanical strain. Ways to avoid this are the over-under method and straight coiling.
Lack of strain relief. A high-quality XLR connector has a strain relief that can take a load of several tens of kg. Other connectors fare much worse in this regard. Someone tugging at a cable without proper strain relief causes a load on the soldered connections, causing breaks at the connection.