Speaking as an ex-Reliability Engineer, there are remarkably few ageing mechanisms in modern electronics. In the past we had tin whiskering in some transistors, and wet electrolytics drying out. Oh, and ESD zaps to CMOS chips often induced a rapid ageing effect causing them to fail weeks or months later. Sometimes a couple of years. Plus, obviously, vacuum tubes, CRTs, VFDs, tungsten filament bulbs, Nixie tubes - they all had well-understood ageing mechanisms.
Nowadays it will almost all be down to the environment (including the operating environment). Thermal cycling can disconnect lead-outs. Mechanical vibration can cause fatigue failures in leaded components. Any humidity over 0% has the potential to cause corrosion of the metallic parts. We should also recognise that most memory technologies have a (theoretically) limited retention time (although back when it was specified as 10 years, many such chips are still doing their job decades later). Exposure to UV, even tiny amounts, will eventually soften the storage elements in a windowed chip. High energy particles from outer space can "soften" the data in EPROMs and flash memory.
Ensuring a device is never operated outside its safe operating area is also very important, but the damaging effects don't just instantly appear on the wrong side of the SOAR line and disappear just inside it, so it is wise to stay well within the safe operating area.
In general, assuming those factors (and others I have forgotten) are adequately addressed, there are little or no detectable ageing effects. This is actually amazing, and almost unique in the world of technology.
By the way, this does not mean they will last forever. Most devices have an intrinsic failure rate, with such failures manifesting as statistically random. Ageing manifests as an increase in the failure rate as time passes. Monitoring the failure rate over a sufficiently large population allows these measures to be meaningful and helpful in deciding when to retire the product being monitored. An aircraft engine, for example.
EDIT: I forgot to mention packaging. Modern plastic packages are not considered as hermetically sealed (at least not by designers of equipment that must be as reliable and durable as possible). This includes spacecraft, military equipment, undersea cable repeaters. Oh, and automotive electronics as well, due to there being some potential safety aspects. Ceramic packaging can meet the requirements for hermetically sealed, which is why such high reliability and durability electronics can be expensive.
Finally, we must not forget the soldered joint. A perfectly-made joint shows virtually no ageing, but even small imperfections can get worse as oxygen and water vapour go to work (plus mechanical disturbance, of course).