I don’t think there should be an official number. I think the English word means something other than age. I think conventional attempts to use it as a car category by age are, well, something that’s up to magazine editors or car parade organizers, but are an appropriation of the word, which actually means:
”1a : serving as a standard of excellence : of recognized value
// classic literary works
// a classic case study on hysteria
b : traditional, enduring
// classic designs
c : characterized by simple tailored lines in fashion year after year
// a classic suit
2 : of or relating to the ancient Greeks and Romans or their culture : classical
3a : historically memorable
// a classic battle
b : noted because of special literary or historical associations
// Paris is the classic refuge of expatriates
4a : authentic, authoritative
// a classic study of eyewitness accounts
b : typical
// a classic example of chicanery
// a classic error”
(from Merriam-Webster https://www.merriam-webster.com/dictionary/classic)