For me, a "classic" is defined as a vehicle that has emotional appeal. It has to appeal to a particular era and it has to have some artistic value. It also has to be old.
My father's old Dodge Dart was a classic to me. So was his old Fury III. And so was the '73 Buick Century, a car I wish I still had today and the first car I ever drove.
My old Aerostar, however trusty and dependable it may have been, and despite the fact that I brought one of my newborns home from the hospital in it, would never be a classic.
It was as ugly as homemade sin.
Even though the 64 1/2 Mustang wasn't exactly original in all concepts, I tend to think of it as a classic. It was the first of a forty-year-long reign of a car that is still popular today, and that popularity shows no signs of waning.