About 3wks to a mo. Or so old
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I had a strain of black game fowl once that every feather, piece of fluff, and leg scale was as black as coal. Even a cross or hybrid made using this strain was always black down to just 1/64ths part of the black blood, regardless if the chick was a cockerel or a pullet.
The only way I am aware of to use feather color to reliably sex young chickens is to breed specialty selected inbred blood lines to produce a bird like the Black, Red, or Golden Sex link chickens. Even that trick doesn't work with the 2nd generation and your birds are back to color chaos.