By the time my chicks are mostly feathered out, they're integrated with the rest of the flock. There simply isn't room in the brooder area for the chicks to stay separated from the rest of the flock for four to five months. Sometimes, the chicks are even younger than that, if we end up with unexpected acquisitions. This year, I ended up picking up a pair of bantam chicks who couldn't have been more than a couple of weeks old. I tried keeping them confined to a brooder, but they were obviously miserable, so I took a chance and let them out into the run with the rest of the chickens. I already had a "chicks only" shelter built that the adults couldn't get into, and the two little ones quickly figured out they could hang out in there and be left alone.
I think it helps that when we acquire new chicks, they go into a brooder area where the adults can see and become accustomed to them. When they are let out into the run, they have plenty of room to run and places to hide if someone gets mean. During the day, we let them out to range, giving them even more space to utilize and plenty to do to alleviate any boredom or stress that may build up in close quarters. The biggest problem we have had hasn't even been with the adults being mean to the chicks, but some of the chicks themselves. Our little roosters have an unfortunate tendency to be rough with the other chicks, and I've had to correct their behavior more than once. Fortunately, a bachelor pen is in the works and things should be quieter once it's finished.