Lot numbers are assigned to all components and to the subassemblies and final assemblies. Changes in a primer for instance from one model to another might cause the upper assembly to change from an XXX to an XXXA1, at least in the U.S. In more recent times, especially noticeable in mortar ammunition, two mortar bodies with the same part number, but assembled with different fuze types, will be given entirely different cartridge numbers. In recent years, lot numbers have included the date of manufacture of the upper level assembly (Cartridge).
Ammunition is assembled for various reasons. In wartime, normal efficient bookkeeping is relaxed somewhat, and things are assembled ASAP. On the other hand, during testing, particular care is taken to try to use the same lot numbers of components, so that consistency is maintained. Special lots of ammo will be made up for the tests. Special inert fuzes are used, inert submunitions are loaded to determine the landing patterns on the ground, etc. There will also be inert rounds loaded up for troop training purposes and for EOD training.
So, yes a lot numbered upper level assembly could contain multi year or multi decade old components taken from inventory, and components that were made the previous week.
In the lastest Gulf war, WWII 50 cal ammo was pulled from storage and relinked and sent to Iraq and Afghanistan. The newly assembled ammo would have a new lot number, but the rounds were 3 times older than the guys shooting them.