The 4th age?

[Manwe Sulimo]

End of 4th: End of civilization as *they knew it* (civilization reformed in Egypt, China, Greece, etc., later).

End of 5th: Fall of Rome (enter Europe into "Dark Ages")

End of 6th: Industrialization? Maybe the colonization of the "New World" (the eequivalent of Aman), or Mankind leaving Arda for the first time ('50s and '60s, with manned missions to space). Of course, if we're at the "end of the 7th age", it'd have to be one of the earlier options.

As for the fourth, all that is said is that Arnor and Gondor were reunited, a lot of Elves left, and Aragorn and Éomer had a few wars in Rhûn and Haradwaith (or at least I think it did). After that, I suppose that civilization declined and disappeared. Perhaps the Dagor Dagoroth happened, and the world was rebuilt in its present form. (Yes, the Valar lost the original blueprints. Typical of them, really.)