OLD GAS ~ Hydrocarbons in the gas react with oxygen to produce new compounds that eventually change the chemical composition of the fuel, leading to gum and varnish deposits in the fuel system. These deposits and impurities can coat surfaces, clog up fuel lines and filters, as well the small orifices in a carburetor (jets) and the even smaller orifices in a modern car’s fuel injectors.
Removing these deposits can be a major job — and your vehicle may not run at all (or run very poorly) until they are removed. I'll 2nd this I've been working on engines ever since I was a young lad even then I figured out old gas wasn't good to run when you've taken apart carburetors and seen the damage old gas causes you wouldn't make that comment. Along with fuel injection lines being clogged.