Emergency Vehicle Lifespan?

At what point are emergency vehicles not allowed to be used for their intended purposes? Is there a standard? Here on the east coast of the U.S. we still have fire trucks being used that are 40 years old, they’re not front line response vehicles but they are used in reserve status but they’re not NFPA compliant. I’ve read in a few ads for used fire trucks that the person or department selling it is selling it because it can no longer serve as an emergency vehicle. I’ve also heard of companies purchasing old fire trucks to use for fire protection on a farm and things like that so it’s kind of confusing because there are no firefighters purchasing the trucks for a fire department but you have a farmer purchasing a 30 year old truck for fire protection uses on his farm even though the seller stated that the vehicles can’t be used for emergency purposes anymore. Thanks for the info in advance.