the problem is, in the Warp there's no such thing as time. Things don't even happen linearly there. Perfect example. in the Chaos Daemons codex, it talks about the birth of slaanesh, and how the warp has no timeline, so in one sense his coming into existence can be tied to a specific time in realspace time, but in the warp he has both always existed, and has yet to be born at the same time. Such is the unknowable horror that is chaos.
Another way to look at it, the warp (more directly, the beings in it) exist as warped reflections of the psyches and emotions of the inhabitants of the material world. So, logically, they have existed as long as life itself has.
I have nothing to back this up with, of course