Well, my flock consist of three differently aged groups, and I let them all range freely. The 8, maybe 9 month birds go the furthest, but still not yet out of my back yard. The 16 week old birds don't seem to go much further away than 20 feet from the run. The 11 week old bantams JUST ventured out of the run yesterday, for the first time. They're not going very far away from the run fence, either.
I'll be putting 8 week old chicks out this coming weekend, but they will be confined in a smaller area to "integrate" with the rest of the flock through safe segregation for a couple of weeks first. Everybody will be able to see each other through the fencing, hear each other, get to know each other but without contact. Then, when I let 'em loose (take down the temporary fence), they'll just join the rest of the flock. They'd be 10 weeks old by then.