It will depend on what you are culling for as to what age to do so. For combs you can do it at day one (single vs. rose). Other comb deformites or issues you will need to wait until puberty. Color probably as early as 2-3 months and stuff like tail I would guess 4-6 months. Culling from your best birds you need to wait until they are adults (14-18 months).
As for what you do with the culls it depends. For me I sell the hens as laying hens. If they have a bad trait or are just a very bad representation I will cull aka use them for food. I usually eat roosters more often than hens. Roosters someone will more than likely use to breed. Cull hens I just give to people who want a back yard flock and have no intention of breeding. I also always inform the buyer why I am selling the bird just so they know why I am not breeding her and to educated them.
Question about missing black pigmentation on one wing:
I have a silver-laced wyandotte hen who, after her molt, came in with all white flight feathers on one wing. No black edging at all on about 6 to 8 of those feathers--just the long flight feathers, not the whole wing.
At the same time my barred rock rooster developed an all-white feather in his tail. Just one feather at the top. Any thoughts?
Background info: They're fed a poultry mash from my local mill plus they get either wheat sprouts or fermented scratch every other day (I alternate the treats and they get only enough that 8 chickens can clean it all up in 10 minutes or so). When there's snow on the ground they have access to an enclosed run that doubles as my compost pile. I put in a lot of fresh horse manure this fall.
Other info: I have two bantam Australorp hens and one went through a molt and all her feathers came in perfectly black, like normal. No missing pigmentation for her.