From what i can gather, the fall of the eldar was when their own... self pride got so extreme that it manifested itself in the warp as slaneesh.
This huge explosion or energy with in the warp was what caused many warp rifts, such as the eye of terror. it also made warp travel good as imposible. this being the age of strife.
and it is after the age of strife that the emporer starts his kick ass comback tour.
so basicly, the fall of the elder was whenever the age of strife started, also the same time slaneesh was born, which does mean that slaneesh is like the seond youngest 'thing' in the 40k universe, the youngest being tau chaps