and a thought: why would the secrets actually only be activated after some time on the world..? sure its an end game feature, but where are the disadvantages, if it was active from the very beginning?
Let me start by saying that the spawn date is configurable and I'm pretty sure it will see some experimentation on different world.
It's not possible, really, to spawn the secrets at world start. Where would you spawn them? The "world" of a TW game only starts to grow with players joining the game. Sure, you could pick some arbitrary coordinates and place secrets there, but who will tell you if the world will actually grow to that size? Imagine a secret placed 2 continents away from the inhabited area of the map!
Three or four month into a world you can see how big it is going to be. Spawning secrets then means they will appear in places that matter. Hopefully places where they spark a lot of conflict too.
Finally, if we spawned the secrets only in the center region of the map it would be an unfair advantage for the early starters in a world. They have the advantage of the early start already, better not pile the secrets upon them too.