Manufactures sometimes decide on how old is too old by fatigue life. This is more closely linked to flying hours but when you run out of life the aircraft is usually scraped or subjected to a very expensive rebuild. The Bulldog is an example, as is the Vulcan.
For a C of A machine the cost of maintenance will increase over time until it becomes a large proportion of the hull value. If you are lucky, at that point the aircraft may be transferred to a permit. An example of this is the Auster. The cost and lack of certified spares meant many were dismantled and stored but a recent transfer to permit has seen a lot of refurbishment projects started using “fit for purpose” uncertified parts and enthusiast labor. Had this not happened then the type would eventually have been reduced to a tiny number of cherished examples operated by very rich owners.
Another issue is an old airframe which is not well looked after. A PA28 140 which has been “sweated” will get to the stage that it is more expensive to get it through a C of A than the aircraft is worth. In the UK there are an increasing number corroding away quietly never to fly again.
Most of the above are reflections on the economic life of an airframe, but this is linked to airframe age to some extent.
Rod1