Interesting fact:

Normally lifespan is measured in time units, such as years, but this is not the case for airliners. Each time an aircraft takes to flight its fuselage is being subjected to pressurization and stress. Therefore, an airliners lifespan is measured on pressurization cycles and mental fatigue which may pose danger. Generally, a “service life of 20 years” is expressed as the approximate figures of 51,000 flight hours and 75,000 pressurization cycles, and long-haul aircraft can remain airworthy far beyond 20 years.