My pullets were introduced to the hens at 9 weeks. I got good advice here that suggested putting them in the coop at night after the hens had gone to bed. When I stuck the pullets in the coop around 8:30 the hens were too "sleepy" to make too much of a fuss. On the second or third night they were all sleeping on the ladder together. During the day make sure the younger ones have enough space to get away from the adults. Mine have two separate runs during the day, but they are able to free range together in the afternoons. We have one old, crabby hen that still chases the pullets in the yard, but there has been no major fighting.