The UK HSE defines ageing and ageing plant as follows:
Ageing is not about how old your equipment is; it is about its condition, and how that is changing over time. Ageing is the effect whereby a component suffers some form of material deterioration and damage (usually, but not necessarily, associated with time in service) with an increasing likelihood of failure over the lifetime.
Ageing equipment is equipment for which there is evidence or likelihood of significant deterioration and damage taking place since new, or for which there is insufficient information and knowledge available to know the extent to which this possibility exists.
This may include corrosion (internal & external), erosion & fatigue e.g. vibration.
HSE guidance is available here.
This topic relates to the following PSM elements:
The Energy Institute has a number of publications associated with Asset Integrity