I agree, it's not a good idea! Last year we mixed day old chicks with week old chicks in the brooder without a problem, I think we just got lucky. This year, the 2nd batch of chicks didn't arrive on time, so the chicks we bought last week on Tuesday grew pretty fast and are to big for the new chicks at a week and a half old. We had to build a divider down the center to separate them as the older chicks started pecking at the head, eyes, and feet of the new chicks. We'll give the new ones a week or so to gain some size and then remove the mesh divider, they should be pretty familiar with each other by then and hopefully we won't have any issues. So I say if the chicks are more then a week old, I wouldn't mix them!