dseretakis,
In the winter in Michigan, around the Detroit area, ALL the roads are salted except for the very few around certain small lakes and in the cases of fresh concrete. The cars that came into my car wash in the winter all were full of snow, ice, mud, dirt and the mostly invisible salt as well.
It was simply a matter of regular care. You could tell just by looking at the car (and the person driving it) what level of care the car received. There were plenty of cars 10-20 years old that were pristine, and these were cars washed regularly, underbody a given, and mostly garaged, too. Remember, sun and tree sap, etc. do a number on a car's finish that compromises its integrity. There were plenty of cars newer with visible rust; damage never repaired, spots not touched up, etc.
While it was certainly ad hoc observation and not a scientific poll, it was very obvious what was happening. Clean cars lasted longer. Good care and solid ownership paid dividends. I saw probably 1,500 cars a week for 6 years, so though unscientific, it was very obvious.