I've brood chicks with a nearly 3 week age gap with no issue....however, there are factors other than interaction to consider. For example, the type of heat used for brooding is a factor. Three week olds require significantly less heat than hatchlings. Using contact heat is the easiest way to work around this. My mama heat pad was easily adjusted so the high and low points were appropriate for the span of chick sizes....and the older birds transitioned to the top rather than in the cave as they no longer needed the heat...the younger birds left the cave at about 2 1/2 weeks because they had their older broodermates to huddle with. A heat lamp does not offer this flexibility.
This is my three week gap group from last year....
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