The age depends a lot on how you are keeping them. If you are at or near the limits of space then they should be near the same size, or about 16 weeks. The more space you give them the younger you can integrate them. Also how you integrate them is a factor as well. If you just throw them in, even at night, the closer to maturity the better they can defend themselves. A slow integration can be done at a lower age. I usually do a slow integration were they grow up in a tractor with a portable run. The established flock sees them from the time I can pull the plug for the heat, but can't physically interact. By the time the chicks are around 12 weeks they can free range together without a problem. That is around the time I can hook the runs together and let them mingle. Depending if I need the tractor for something else or not I may leave it that way until they start to lay. then move them to the main coop. and hide the tractor. So the age can vary when you integrate them. To give a more definite answer we just need a little more information.