I have never heard of ageing effects in copper or polyethylene - or
ancient ebonite spacers even.

The very first coaxial carrier communications cable was laid in Great
Britain by the Post Office, around 1937, between the cities of
Manchester and Leeds. There were 4 coaxial tubes inside a lead
sheath. Outer conductors = 0.375". Inner conductors = 0.1", which
later became the standard. Mostly air spaced. Inner conductors were
supported by ebonite disks or similar material, spaced at about 1.5".
Polyethylene was still waiting to be invented. Working frequencies
from 60 kHz to about 2 MHz. Repeater spacing about 5 miles.

Around 1960 I had the opportunity to test sections of this cable. As
far as I could judge it was in perfect working order. Bear in mind it
is possible to detect small changes in attenuation only by looping
back on very long lengths. It cannot be done in the lab.

I imagine coax, with temperature expansion and contraction, very
slowly 'breathes' through the ends and draws in humid atmospheric
pollution. Perhaps after 50 years it may have some minute detectable
effect on attenuation and appearance. Attenuation is the last
parameter to fail. Far more serious things have to happen to a
transmission line before loss becomes noticeable.

For example, a coax line can be almost flattened with a hammer over a
length of several feet which will make a shocking mess of impedance.
Yet, provided the inner and outer conductors are not in contact with
each other, additional loss will be undetectable.

=========================

Nothing happens to metallic copper with frequency. But copper
conductors also have internal inductance in addition to conductance.
Inductive reactance increases with frequency. The increase in
inductive reactance begins at the centre of the conductor and drives
the current outwards towards the surface or perimeter. At
sufficiently high frequencies the current is forced to flow only on
the conductor's skin.

The conductance of copper remains the same. But the cross-section of
the conductor allowed to the current is very much reduced and so the
effective resistance per unit length increases together with the
inductive reactance.

It's an interesting fact that at frequencies where skin effect is
fully operative, conductor inductive reactance and resistance become
equal to each other. Measure one and you also know the other.
----
Reg, G4FGQ