re: OPF
After every flight, the Shuttles get rejuvenated inside the OPF and ready for the next flight. Well, rejuvenated to some extend, perhaps.
Just like it was difficult to say how old Mir was (some modules arrived in 1996 while the first was launched in 1986), th'e Shuttles have engines that are younger than they are, more recent tiles, etc..
Raoul