From what I've read, you can introduce them younger if they are free ranged. Then there is a lot of space and other things for the flock to focus on so they don't pay all of the attention to the newbies. If its a coop/run environment, I'd say introduce around 10 weeks when the younger ones have a bit more size to them. I tried to introduce eight six week olds to my 13 week olds and it was a disaster-even though they had been exposed to each other for a week (the younger ones were in a dog kennel in the coop while the older ones had free access). I'm now waiting until next weekend to try again.