You will probably get lots of answers to this. Here is mine (based on experience and what I know of electronics - I'm not an engineer).
Tube gear will, of course wear out pretty quickly but the tubes are replaceable. Power amp tubes may only be good for a year or two (with daily use), preamp tubes will last much longer.
Other tube gear parts will tend to go a bit faster than solid state stuff because of higher chassis temps. This will include capacitors (they tend to dry out). However, we are now talking ten or twenty years or more, depending on the design.
In theory, solid state stuff may go along indefinitely without "wear" - including caps. I have an old Denon receiver that has had daily use (first in my wife's business office and now in her hobby room) since the 70's. No problems.
Many pots (volume controls, etc) will get noisy over time but can usually be cleaned and returned to like new status.
So most gear will last for a very long time and, if SS, may need zero maintenance. It basically won't "age" in that sense. There is even a school of thought that believes some gear gets better with age - even after the initial break in period. I couldn't say.
