skipness
the diff now between the 21 yr old machine and the 9 year old machine will be negligible, from a powerplant perspective, since all the engines will have been through a repair/refurbish at least once. A refurbish is never going to recoup 100% of the fuel burn capability of a new manufactured engine, but will get to within about 1-2% of the original figure. There will also be some slight aerodynamic deterioration on the airframe.
arrowhead
it's 90% down to your engines, we are just studying our fleet of 320/321's V2500 engines and they lose about 1-1.5% fuel burn a year. When you put them in the shop for perf restoration they come out within about 1.5% of a new engine.
Regards,
N1 Vibes