Depending on breed size and weather I usually start putting mine in a round dog pen in the actual chicken run around 8 weeks during the day. At night I have a segregated pen in the coop that I put them in so they can all see each other and get used to each other I do this for 2 weeks then take down pens
It depends on your coop size, set up and weather. I move my chicks to the coop brooder within a week after getting them. The first week I have them in my garage brooder, so I can keep a better eye on them. Raising them in the coop with the big girls is an advantage to integrating, because they are used to seeing each other. At 3.5 weeks I left my brooder panic doors open so the chicks could interact with the big girls. Yesterday 4.5 weeks old, I took the brooder down and they are fully integrated. 3 - 4 weeks old has been the easiest age for me to integrate chicks with older chickens. I have seen some minor pecking but nothing to worry about.