Age
The Classic Car Club of America requires cars to be built before 1948 to be considered, but many states license cars older than 20 to 25 years old as antiques.
Price
Classic cars depreciate until a certain age, just like any other. What sets them apart is that they depreciate less rapidly and then begin to appreciate until the asking price is frequently anywhere from 10 ro 100 times more than the original price of the car.
Distinctive Style
Innovative and attractive styling always sets a vehicle apart, no matter what its age.
Historic Preservation of Original
Classic cars, many of whom may have "done time" sitting in the "back 40" or as teenagers' street cars, must be comprised of original or restoration parts: they may be "over-restored" but not modified.
Distiguishing Engineering
Most classic cars were recognized--and continue to be admired for--innovative or trend-setting engineering.
Desirability
The value of a collector car is in large part determined by its desirability, which is largely determined by how many were built and how many remain.
Documentation
The provenance of a classic car must be clear--it should contain all sales contracts and maintenance records.