Hens lay from around 20 weeks of age. You will notice that hens pullets not layer have yellower legs and around their eyes. Once they begin to lay, the yellow that was going into their system (so legs, etc) goes into the yolk of the egg. So keep an eye on that for signs. The comb also becomes much fuller and redder when they are coming into lay.

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?? about the yellow legs and the yellow moving to the yolks. Never heard of that and would certainly not apply to black legged chickens. If anything the colour of the legs do fade as they age in both males and females.