Once a bird has reached maturity at around 6-8 months old, meaning a female has reached laying age and the males are crowing/mating and all mature feathers have fully come in that is when I would cull for white (if you have other birds of good quality to choose from with no white). Mature primaries are rounded on the ends, juvenile primaries are pointed on the ends. I wouldn't cull juvenile birds for white, as most juveniles will have some white in the primaries. White primaries in juveniles molt out and usually the mature primaries come in black (or blue in the case of blue birds).
However, I like to see very minimal white in the wings of my juveniles. I am happy if I only see 1 white feather in each wing and even happier if there is no white in the juveniles' wings. Most juveniles will have some white though. Some of my juveniles have anywhere from zero to 4-5 white primaries in each wing. I don't like seeing very many white feathers in my juveniles just as a personal preference as it seems to correlate with carrying white and showing up in the adult plumage. I am still testing that theory. I do document how much white juveniles have in their wings, and I even document how much white they have on their faces and down as chicks as I am wing banding them.
The set of feathers that the birds develop as they reach sexual maturity at around 6-8 months old are the set that they will have until their adult molt. The first adult molt typically occurs at around 12-18 months or so. Most breeders I have talked to cull birds that show white at 6-8 months old, because more than 1/2 half inch of positive white on any one feather or more is a DQ. They cannot show the birds, nor do they wish to breed the DQ further. If a bird has nice qualities otherwise, I have been advised to pull a white feather if one is found by 8 months old and see if it comes back in black. I will be testing that theory and will post back with my results.
I will also note that if a bird is exceptional in other aspects and has a white feather, if mated to a bird that does not have the same fault the bird could still be worked with. White is a fault just like any other and can be improved upon. I would only cull a bird that showed white if I had better birds to use in place of them. My favorite hen, Queen has one white primary feather. She has the best type of any other Marans hen I have. I wouldn't dream of throwing that away. When you have the luxury to cull for white, that is one thing. But don't throw out an otherwise good bird for one white feather. That is my 2 cents.