Putting old gasoline in your car can damage your car and cause things such as clogged fuel lines, clogged fuel pump, clogged fuel injectors etc.
Old gas that has turned to varnish or has begun to turn to varnish will cause problems with your vehicle so do not ever put any old gasoline in your car.
The old gasoline when put in the cars gas tank will make your car run rough, stall, poor idling, poor acceleration etc.
The car may also not even start if you just put the old gas into the cars gas tank when there was not much other gasoline in the tank.
So always make sure you put fresh gasoline in the car to avoid problems.
Gasoline will usually last in a cars gas tank for a few months before it starts to degrade so the gasoline needs to be used before it goes bad.
It's best to keep your gasoline tank full but also drive the vehicle and use some of that gasoline up and then put fresh gasoline in to keep the gasoline fresh and avoid clogged fuel injector, clogged fuel filter, clogged fuel pump, clogged fuel line etc.