The age of an ATR series aircraft like the -42 or -72 is determined not by the year it was manufactured, but how many ‘cycles’ it has logged. One cycle is defined as one takeoff and one landing. For example, an aircraft is considered ‘old’ if it has performed many cycles, and necessarily because of its chronological age.
An older aircraft may not have performed too many cycles, and can keep flying as long as it is maintained as per the requirement of the manufacturer. A typical ATR-72 has tens of thousands of parts, each with its own lifespan measured in cycles. For a plane to be considered airworthy, each one of these parts must be maintained or replaced as per the manuals. Just repairing a particular part is not adequate.
It is not just the moving flight control systems of a plane, or the cockpit instrumentation that need to be checked. An aircraft’s wings, fuselage, flaps, ailerons, tail assembly, all need to be thoroughly checked after a certain number of cycles, every rivet and joint must be carefully inspected.