In the real world aircraft age is measured by two things:
Hours flown and cycles flown. Hours is obvious and cycles refers to a completed flight, ie a take off, some time in the air, followed by a landing is one cycles. Most component life and inspections are specified by either or both of these parameters.
These two parameters can be almost in sync to wildly different. The best example of the latter is JAL & ANA's B744D. They were delivered without winglets and configured for Japanese domestic operations. In this role they will obviously rack up a disportanate number of cycles for the hours flown. So they brought the winglets and as the airframes reach a high number of cycles they will add the winglets and reconfigure them for long haul ops where the hours will catch up with the cycles.
BTW have any airframes been so reconfigured yet?