I have mine in (9 wk chicks) but I blocked them off and will open them around 14 wks.
My thinking is that I didn't want them to begin sleeping in the boxes and get them poopy in the process. When they're young they'll all gather in the nice cozy box. I wanted them to go to the roots first and be very well established that they are the place to sleep (and poop)!
When I open them up, I'll put in some ceramic eggs (could use golf balls or other egg shaped items) which is supposed to help them get the idea that's where the eggs go.