Lufthansa has around 50 airbus narrow bodies over 20 years old, with Swiss and other group companies probably operating another 50. So it is not correct to say that after ten years they are basically only worth parting out. What makes a great deal of difference is how they have been maintained. Dodgy third world leases with questionable maintenance records are what destroy value. Operated and maintained properly, then 25 years and more is a realistic life for such an aircraft.