The answer to this might depend on whom you ask, since the definition of "classical music" vs. "pop music" is a contentious subject. One could argue that it was not until the decline of Tin Pan Alley and similar industries elsewhere that pop music really became something separate, because until then, even the "popular" styles of music were defined by their written-out, published form (and this is one definition of classical music). At the opposite extreme, some could say that the distinction has existed from time immemorial, by arguing that many forms of traditional or "folk" music are in practice indistinguishable from pop music, at least as far as the origination of any single piece is concerned. (In a folk-music culture, these pieces then undergo a process of ongoing 'remixing' and re-elaboration which defines folk music as we know it.)