That's
an answer, but probably not the correct one. Tolkien says (found in Letters) that the ages are now passing more quickly. The 4th, 5th and 6th, according to him, would not have been as long as the previous ones.
In order to make Tolkien's timeline work, and for Aragorn, his descendants and opposing kingdoms to eventually assimilate into our own world, I believe you would have to do one of the following:
- forget about prehistory, and begin the 5th age with the civilisations of Sumer
- introduce a cataclysm like the Biblical Flood to separate our times
Obviously Tolkien's history of our world is just an alternative, and not the real story. At the time of writing, I don't think people knew nearly as much as they do today about Darwinism, the missing link, and prehistoric societies. From his writing, I think that we can infer Tolkien didn't really care much for that sort of thing. And so his 7 Age timeline pretends that it never happened!