I think the original explanation was that the warp storms were caused by the Eldar (either due to them surpressing their emotions in the original version or by their excesses in the current version). Wiping out most of the Eldar is what carmed it down.
Sometimes Slaanesh is blamed but the sort of Jungian combined psychic unconciousness of the pre-fall Eldar and the embyronic form of Slaanesh are pretty much the same thing.
Even now Slaanesh still isn't fully detatched from the Eldar, hence the soulstones and the Dark Eldar having to top themselves up as their souls naturally get pulled into Slaanesh.
The height of the Eldar and the Dark Age of Technology were more or less concurrent. The Iron Men thing happened way before the birth of Slaanesh. Since as I say above, the fall into decadence of the Eldar caused the warp storms that cut off terra, so the Human and Eldar golden ages end at pretty much the same time, though its a bit less dramatic in the Eldar case as its more of a cultural decline than everything suddenly going bad.
15th millenium
Dark Age of Technology starts, humans colonise the galaxy and have the iron men as their army
25th millenium
Warp storms start to rise, ending the Dark Age of Technology and causing the Age of Strife. The Iron men have already been discontinued by this point.
30th millenium
Fall of Eldar, birth of Slaanesh, Eye of Terror forms. Great Crusade begins.
41st millenium
Current timeframe.