I have seven week old Dark Cornish chicks and i want to keep some as breeders.. At what age will they be developed enough i can cull out the freezer birds from the keepers?
Varies somewhat by line and conditions.
But what I would do is get some leg bands, and mark the birds of both sexes that reach butchering size first, so you can distinguish them later. Often times the birds that mature to a smaller adult finished size will mature quicker, but not always. Usually the truly big boys late longer to get there, so keep that in mind too.
If these are just for your own use, then you can decide what appeals to your eye beyond carcass development, but I would recommend looking at pictures of good quality Cornish to help you get a feel for what proper type is. Especially if you are entertaining thoughts of maybe selling some down the road.
I know I didn't answer your question, but there are several variables to consider. Some things like maybe bad feet, or crooked bill, would put some one on the freezer list as soon as big enough. A crappy personality will do it as well for me, especially on a male.
You will have to weigh your options, like if it's worth waiting for a little extra final size, or go with a smaller, quicker to mature male to head up your breeding pen. I put extra effort into the male as every chick will carry his genes. The females you have a little more flexibility to experiment with, and you can keep next years breeders from the ones you guessed right.