Its more than the 6000 years that JRRT alluded to, but i always imagined the 4th age ending before the last ice age (little ice age around 15-20K years ago..?). In my tragically derainged mind, this would explain the lack of evidence today of any of the civilizations of pre 5th age ME. Or at least the mid to northern sections. Getting scraped away by a preceding and receding 2 mile thick sheet of ice would do that, no?
