An aircraft is officialy "born" on its rollout. From then, it is like a human birthday.
Aircraft have flight logs in them that record the number of hours they've been flown. This is usually the marker used to determine the "age" of an aircraft. If the airframe has not been stressed because it sat around in a hanger, it has low hours and is nearly "good as new" from the point of view of its air worthiness. Also important for the larger aircraft is the number of takeoffs and landings. These determine the number of cycles of decompression and recompression with the ship's achievement of high altitude in transit, and then its return to the "normal" pressure on the ground.
It is the number of flight hours on a craft, and the number of decompression-recompression cycles (on the larger airframes) that "age" an aircraft. Inspection, maintenance, and rebuilding cycles are all based on the hours on the airframe and engine(s).