The MPG of the vehicle does decrease with age.
The older the vehicle gets the more wear and tear on the engine and other components such as ignition coils, ignition module, spark plugs etc.
This then causes the engine to burn more fuel to cover the same distance.
Although if you maintain the vehicle properly and change spark plugs, spark plug wires, ignition coils, etc then the vehicles MPG should not decrease all that much with age.
No matter how well you take care of your vehicle, it's engine efficiency and power are never as good as when you drive it off the lot.
However, while a product of the miles you've driven, this decrease in efficiency is most likely due to faulty or worn engine components.
A bad fuel injector or dirty/old fuel filter can drastically affect the flow of fuel into the engine.
A fuel system problem is one of the most common causes of poor gas mileage.
The more you run your A/C, the lower gas mileage you will get.
Aggressive driving (speeding, rapid acceleration and braking) can lower your gas mileage by roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic.
Excessive idling decreases MPG.
The EPA city test includes idling, but more idling will lower MPG.