I think that "classic" is like "science fiction" - it's whatever you're pointing at when you use the term.
Seriously, though, I think that many people use "classic" when they actually mean "classical", as in "from the 'olden days' like that classical music". This is why many of the so-called 'classic' names from the SSA data leave most people cold: the data is more modern than is necessary for an accurate depiction, and names such as
Nina might not be in there if there was another century's worth of data to consider.
I see "classic" currently being used as an (unintentional) synonym for "Regency", "Victorian" and "Edwardian" (and whichever
George was around in the early 1900s, I can never remember lol). And it is most often applied to Biblical names and, for girls, flower/plant names.
I'm not sure that there *is* one definition or an empirical system that would be acceptable to everyone. It's very much a subjective description rather than a discrete category.
♦ Chrisell ♦
All we have to decide is what to do with the time that is given us. - J.R.R. Tolkien.