Do you still have the original hens? Are they still laying? If so, the youngsters MAY follow the older hens, and mimic their behavior. If you have a run, you might just take a "wait and see" approach. Put some fake eggs in the nest boxes, make them nice and comfy with lots of hay for them to "rearrange to their liking". Or you could take a preemptive approach. Wait until you see signs that those first eggs are due to arrive (squatting for you, or being bred by a mature rooster, singing the egg song, or spending a lot of time "playing house" in the nest boxes. When you see those signs, block them into coop and run. This is one reason why it's important for your birds to have a run, even if you let them free range EVERY day.