I think we'd have to differentiate what's considered classic from genres themselves. Rock's been around for about 70 years, so it makes sense that a classic era emerged about 20 or 30 years after that. You could see it as something of a golden age of the music. Punk started in the 70s, so if you want to call Green Day classic punk (although no one really uses that term) I think it'd fit.

Similarly if someone said classic ska I'd be thinking of 2nd wave, like the Specials, which is also about 20 years after the start of the genre.