The total life of an aircraft is usually based on a combination of fatigue life factors. There is a design requirement for an aircraft to reach a certain number of hours or cycles (the design service goal) and that is partly based on the mission mix the aircraft is assumed to carry out throughout it's life. So maybe 70% is cruise profiles, 20% is low level profiles and 10% is dogfighting. If the more ardous profiles exceed their assumed percentage then the total life in terms of hours (or cycles) can come down from the design estimate. Throw in to the mix that some parts are replaced after a certain number of cycles and at different times, or if an envelope is exceeded and you generally become limited by a part of the structure you can't change eg a wing pivot or a wing skin around a landing gear bay. There's lots of work done to extend service goals if the demand is there, and the manufacturers can go to great length to replace parts that are not meeting the design service goal.