I also raise them outside in a wire pen in the run. I start mingling them under supervision when they are 3 weeks old and I have total integration with the flock (and the brooder completely removed) by the time they are 4 weeks old. I have found that done this way, the adults don't see the chicks as a threat to their space, food, water, and established pecking order the way they see birds closer to them in size. Any pecking that's done is usually just a reminder to the chicks that they are overstepping the boundaries of good behavior and don't really amount to anything except a good lesson for the little ones.
After getting the MHP thread started, other folks started brooding this way. When
@azygous posted about her portal doors, I knew I had to add them to my outdoor setup and they have been a lifesaver for my chicks and for me ever since. Before that I was just propping one side the brooder open big enough for the chicks to get back into the brooder and too small for the adults to follow. But chicks ain't always real bright, and they'd sometimes run the length of the brooder, make it almost to the open place, then panic and start running back the way they came from. Grrr. I did also have a big half log which was hollow in the center, with the hollowed side down, and they ducked under that if they were too far from the brooder to get away from the occasional adult who was getting a little too pushy. Thank goodness for that and the addition of the portal doors! Once I put those in, I wondered how I'd managed the broods before that out there!
BUT that's just the way it's worked here for me and the batches I raised this way. A lot of people would rather wait until the chicks are about the same size as the adults. It's whatever works for you, your birds, your setup, and your situation.
Chicken behavior is fascinating...and usually pretty doggone entertaining as well! My favorite thing was when something new would go into the coop or run. They'd huddle into a corner and scold this intruder like it was alien coming to take them away! They'd stare at it like the "stink eye" would make it go away!