What you are talking about is highly theoretical, that there might be a molecule of jet fuel that grows old in the tank. Consider the reality.
When the plane is serviced there may be five to twenty thousand pounds or even more remaining in the tanks, depending on which type aircraft we are talking about and some other variables. The fuel remaining is normally cold soaked to well below the freezing point of water. The fuel being added is tankfarm temperature. Presumable they will not mix until the temperatures come together. I think this is called "affinity" in chemistry.
Eventually the old and new fuel will mix. After all, it is being agitated constantly once the plane starts moving. The notion that some fuel will never take its turn down the pipe and somehow "age" is just not worth considering.
Here is a concern! When you drain fuel from a tank, it is replaced by air. Fuel down, air up, fuel up, air down. The airspace above the fuel in the tank increases and decreases. Seen another way, the tank is breathing. Now all air has some moisture content. Even in LAS
there is some humidity. What happens to all that water that is drawn into the tanks?
You can be sure that some very smart people have addressed both our questions.
Yours is a little like the question that goes: If electricity is the flow of electrons, why does electric wire age since the matter within it is being constantly replaced?
Happiness is not seeing another trite Ste. Maarten photo all week long.