A good starting point for when a car becomes classic is the age at which it can legally sport an antique license plate, 25 to 30 years, depending on state law. However, many factors affect whether a car is classic.
Creating a precise definition of "classic car" when it is used colloquially is much like trying to define the slang word "cool." There is no guarantee as to when a car becomes classic or even if it becomes classic at all, and opinions on the matter vary widely. The Classic Car Club of America, for example, has a very strict definition, narrowing classic cars down only to "fine or unusual motor cars which were built between and including the years 1925 to 1948." This definition, however, excludes models like the 1964 Ford Mustang and the 1950 Volkswagen Bus, cars that are almost universally considered classic.