Coolant doesn't last forever. You have to change it every 24,000 miles (green coolant) or 100,000 miles (extended-life coolant). If you keep driving on worn coolant, expect to replace the radiator, heater core and water pump.
An easy avoidable “oops”
Green, yellow, red, orange and blue coolants create cool “mud” when mixed. Cost of a new heater core: $400 to $1,000.