To be considered a classic, a car must be more than 20 years old. It must also be in good condition and maintained in a way that stays true to the original design. Additionally, the Classic Car Club of America has a list of models that are considered classic.
Older cars, even ones considered classic cars, can also be labeled as antique. Antique cars are normally more than 45 years old and stay true to their original design as well. An old car that has been heavily modified may be considered vintage, but it can never be classic or antique.