I think we can generally say the idea of history and pre-history isn't necessarily "real", but rather is discursively produced in particular social or disciplinary circumstances. For example, we could put the dividing line at "literacy" -- i.e., prehistoric era are those before written documents. This tended to suit the history discipline when historians almost completely relied on written sources for evidence. Now that we tend to often combine archaeology, historical linguistics, oral tradition, and other kinds of human traces as well as representations, this dividing line isn't necessarily as useful anymore.

I once saw a very interesting lecture by the Italian scholar Luigi Cajani, in which he argued that the notion of prehistory was an enlightenment way of "othering" a kind of barbaric people -- those who lived in the past -- alongside racism, technologism, and other ways of "othering" people who weren't modern, bourgeois western Europeans. In other words, the bourgeois European middle classes tended to define themselves by defining others as not being civilized like them -- whether the lower classes, or Africans/Asians/Americans, or Irish & Bretons, or ancient peoples. He was quite convincing in his presentation.