Older hens teach junior cockerels manners. They won't allow him to mate with them until he is big enough and firm enough to overcome their objections. Once he's done this, they'll allow him to mate. It is not unusual for flocks of differing ages to free range in their own groups. I have a multi-generational flock, and they range in four or five groups, according to which birds they were raised with as youngsters. They all get along fairly well, but that's because they have plenty of room to work it all out. All of them return to the same coop at night and roost with no problems. I only have one rooster as well to my 31 hens and 3 pullet chicks.
Watch your drake and don't let him mate you hens. He could injure them.