I always put them out at 5 or more weeks, but I would recommend housing your newest chicks in a large metal dog cage in the coop. This provides them with a place of refuge in case the older birds are picking excessively. I'm pretty sure the logic behind this method is so that the elders can get...
I'm not an expert, but from personal experience, the girls lay fabulously without a roo around. Our number of eggs averages about 17-18 a day from 19 layers (even though we have 12 more who haven't started), so I would say we get respectable production from the chooks.
-Alex