Purplehelmet:
The manufacturer will normal set the airframe life and after consulting FAA or CAA etc for their input.
For most aircraft it's pressurisation cycles that normally effect the end life of an airframe. Hours will just give you a jaded aircraft and as previously mentioned cost of maintenance and upkeep and high fuel burn normally sends it to the desert... The long range companies tend to average high time but lower cycles hence many old B747s and the like are still trucking, but take the short range types doing hops the hours flown are lower but cycles are very much higher and life tends to end quicker.
A famously reported Aloha B737 was lowish hours compared to the cycles and felt fatigue badly.
TW