Skyhigh guy brings up a great aside to this conversation. Cycle lives of aircraft and what can happen when that is exceeded.
The Aloha Airlines 737 loss in 1988 had almost 90,000 cycles on an airframe that had a DSO extension of only 75,000. The loss of one of the cabin crew was particularly tragic, considering the aircraft was landed successfully with a huge chunk of the upper fuselage having gone missing.
The DC-9 has one of the highest DSO (design service objectives) in the business. It was originally rated for 40,000 cycles but in service cycles have exceeded 140,000 landings for some aircraft.
The second rated aircraft for durability in the same category would be the 737NG, which has a DSO of 75,000 cycles, which through a moderate increase in maintenance costs can be extended to over 100,000 cycles.
The next in line for short haul aircraft would be the Airbus A320 series. The DSG (design service goal - same as DSO, different nomenclature) for this aircraft is 48,000 with an extension programme (ESG) allowing for 60,000 flight cycles, again with a (more or less) moderate increase in maintenance costs. There is talk of a stage II ESG that could take an A320 to 75,000 cycles.
I've been told that most commercial aircraft can be kept airworthy for an extended period of time if money is no object.
Only when major components are showing signs of unrepairable fatigue, such as spars, keel beams, other internal and difficult to replace structural items are the aircraft forced out of service by the regulators. The ancient Grumman flying boats used by Chalks in Miami is an example of this limitation.