What is the Optimal Time Period for a Commercial Airliner to Remain in Service?

The recent un-contained engine failure on an aging United Boeing 777-200 raised the question of whether this airplane was simply too old, as it appears that an engine blade sheared, potentially from metal fatigue.  While it is not often that even older aircraft experience fatigue issues that result in serious problems, older airframes do experience higher maintenance costs than younger aircraft, as well as more frequent flight cancellations.

On average, we know that older airframes tend to have increased maintenance costs after about 12 years in service.  A rule of thumb in the industry was to add 1% to 1.5% per year for each year after the twelfth year.  In addition, as maintenance incidents increase, one can very conservatively expect additional flight cancellation due to maintenance after 10 years, and double that after 15 years, and treble it after 20. Flight cancellations are expensive, with re-accommodations for passengers and, if mechanical under the control of the airline, additional costs for hotels and meals