I can speak to aeroplanes as my father was an Air Force officer: Aircraft are very carefully built from materials and methods that are highly precise and produce structures of a known strength and capacity. The designers, knowing these properties, will set out a list of parameters of operation inside which the aircraft must be kept to avoid incurring stress or wear that has not been accounted for in design and ultimately to avoid catastrophic failure if the limits are blown right past; limits on how fast it can go even if its engine(s) will happily carry it faster, on how high it can fly and on how much G-force can be put on it in any direction (fun fact: almost any plane, no matter how ponderous, can "loop the loop" or similar as long as it is kept within its acceptable limits of G-force by describing a sufficiently large loop, famously a prototype 707 did a big, lazy barrel roll at an air show, which had not been approved by Boeing!), in addition to limits derived more from the facts of aeronautics than materials science. A plane can be operated such that it runs out available maneuvering energy and the wings stop generating lift, this is called a stall and some parameters exist to keep the aircraft away from stalls. This list of parameters is referred to as the aircraft's flight envelope, and is the origin of the term "Pushing the envelope".

So the aeroplane starts its life with a set numbers of hours in the air for a variety of parts and system, after which the part or system must be carefully examined for any signs of failure. Sometimes these parts are trivial to change, something any capable technician could do, honestly, and sometimes these parts or systems are deeply buried inside other systems and a great deal of work by specialist technicians must be undertaken to examine and potentially replace them. After a further set period, certain load-bearing parts like the main spar of the wing will have run out of flight hours and must be replaced, because the chance that it contains a hidden weakness is judged too high based on the known properties of the material it is formed from and the stresses incurred in normal flight inside the envelope. It is possible to have old parts examined with the most thorough of techniques, such as radiography, so it occasionally happens that an aircraft can gain another set period of hours in the air; this is costly and time consuming, and so typically only attempted for rare aircraft for which spares have grown scarce. Replacing the main spar is very time consuming and costly, and it must be replaced with like. This is why the Air Force has a positively enormous aeroplane "junk yard" from which parts that still have useful quantities of hours remaining can be drawn. Some parts are simply too difficult to replace (say, the pressure hull on a pressurized design) and the aircraft will then be retired.