If motorcycle owners were to adopt the definitions used by the AACA and CCCA, the classifications would be as follows:
1. Antique Motorcycle: a motorcycle which is at least 25 years of age and has been restored to original manufacturer specifications. Similarly, many states which issue Antique tags required that the vehicle be at least 25 years old, though in other states the age limit is a less stringent 20 years. Some require that the vehicle be maintained solely for exhibition, parade, and public interest usage.
2. Classic Motorcycle: a fine or unusual motorcycle built between and including 1925 to 1948. This would be the definition if motorcyclists applied the CCCA's definition for automobiles to motorcycle. Obviously, it is narrowly focused and highly exclusive, and it's probable that few motorcycle enthusiasts would agree with this definition. After all, there are many, much newer bikes which would almost certainly qualify as classics in most collectors' minds, such as the Honda CBX, Triumph Bonneville, Norton Manx, etc.
Obviously there are issues with these two definitions, and they all but ignore the term "vintage motorcycle." However, there does seem to be a recurring theme among definitions: an antique can qualify as such based solely on age, while a classic requires a higher level of desirability, scarcity, or just plain je ne sais quoi.
Perhaps a simpler, easier, and more liberal set of classifications would be those employed by the Vintage BMW Group, which are completely age-based:
Antique Motorcycle: 1923-1945
Vintage Motorcycle: 1948-1969
Classic Motorcycle: 1970 onward, but at least 25 years old (1970 to 1985)