My brooder is in the coop, it has electricity. The chicks go there straight from the incubator or post office. Since they grow up with the flock and I have lots of room inside and out, when they hit 5 weeks old I just turn them loose. They are basically integrated. If you don't have a heat source in the coop you obviously can't do this.
In the heat of summer I've had broody hens wean their chicks at three weeks, just leave them entirely on their own to make their way with the flock. Even at three weeks they can manage. Mama hen has spent three weeks introducing them to the flock. And I have a lot of room, an 8x12 coop and over 3,000 square feet outside. I keep repeating that because I think how much room you have inside and out is very important. My flock typically consists of a mature rooster, some mature hens, and often has two or three different aged chicks with the flock. Some of those chicks are on their own, some may be being raised by a broody hen.
My suggestion is to house them side by side across wire when you reasonably and practically can. That will differ for different people. After one or two weeks turn them loose when you can be around to observe. After that, base what you do on what you see, not what some stranger like me over the internet tells you that you will see.
Some generic suggestions. Give them as much room as you can. You can improve the quality of what room you have by adding clutter. Clutter is things that break line of sight and they can hide under, behind, or over. Have widely separated feed and water stations to avoid conflict. Don't try to force them together in a small space, thinking you can force them to get along. It doesn't work that way. If they want to sleep separately, let them.
It is possible they will merge into one flock pretty quickly but it is also possible they will form two sub-flocks, the younger avoiding the older until they all mature. Then they will merge. My goal during integration is that no one gets hurt. That's it, no one gets hurt. The merging into one flock will happen later when it happens.