How do jetliners age (major or notable checks and part replacements)?

As commercial airliners can be (and are) operated for many years (or even decades), I've started to ponder - how do they actually age? Or - in other words - what are major or notable maintenance-related events (checks/replaces) during airliners' service life and how often do they happen?

While there are many parts that come and go every $$x$$ hours of flight or $$n$$ takeoff/landing cycles that are hard to list, my first guess was the engines or on-board computers, but then I thought about the fuselage. I assume that they are designed to withstand whole service time, but at the same time material fatigue seems to be unavoidable, and this holds for many other parts.

If that's possible, I'd especially like someone elaborated on Boeing 737NG and/or Boeing 747 (differences between the two - or lack thereof - might be especially intriguing), but any small to long range airliner will work.

If that question needs fixing/clarification, I'll be really happy to do so!

• You should look up what an "A/B/C/D check" is in regards to aircraft. – Ron Beyer Jan 28 at 20:34
• On the system side, read about MSG-3 sassofia.com/blog/…. Under MSG-3 most system components can be run indefinitely because they are periodically performance tested for degradation. For example, a hydraulic actuator that passes functional testing for internal/external leakage every C check (5-6 thousand hours) can run for the entire life of the airplane theoretically. – John K Jan 28 at 22:24