My 7 chickens have 3 different hatch dates, and the oldest is 17 wks, with a couple at 16 wks and some more at 15 wks. I didn't have any trouble at all with the age difference in mixing them in the brooder, and they all get along just fine. There was a slight pecking order issue with the addition of each "batch" but nothing horrific at their ages, then.
