The theory is that items critical to the safety of the aircraft are replaced over time, so that essentially you never get an old aircraft. The components requiring replacement, be they structural or equipment,are defined in the approved maintenance program. So in theory you could have an aircraft with unlimited life, except the cost of the repairs and spares,eventually exceeds the cost of replacement aircraft. All over the world you'll come across venerable examples of "old" aircraft that have still found a usefull niche. There are 50 year old DC3's in service on every continent, just depends on what it costs to keep it flying and what you get paid when you fly it. I was in the US desert last year when United flew in 10 year old 747-400's and said they were done with them. It's all economics..............