Ok let’s go with four months for the cockerel and three months for the pullets.
Typical mating behavior between mature consenting adults.
The rooster dances for a specific hen. He lowers one wing and sort of circles her. This signals his intent.
The hen squats. This gets her body onto the ground so the rooster’s weight goes into the ground through her entire body and not just her legs. That way she can support a much heavier rooster without hurting her legs.
The rooster hops on and grabs the back of her head. The head grab helps him get in the right position to hit the target and helps him to keep his balance, but its major purpose is to tell the hen to raise her tail out of the way to expose the target. A mating will not be successful if she does not raise her tail and expose the target. The head grab is necessary.
The rooster touches vents and hops off. This may be over in the blink of an eye or it may take a few seconds. But when this is over the rooster’s part is done.
The hen then stands up, fluffs up, and shakes. This fluffy shake gets the sperm into a special container inside the hen near where the egg starts its internal journey through her internal egg making factory.
With three or four month olds you are not dealing with consenting adults. You are dealing with adolescents (or soon to be adolescents that have no control over their hormones. Cockerels normally mature earlier than the pullets and are being driven mad by their hormones. The pullets have no idea what is going on so they certainly are not going to cooperate. The cockerel is getting ready to enter puberty. That’s probably why he is available. The pullets are still probably a couple of months away, when they should start to lay.
You can try introducing him now. They might get along fine. But definitely have a plan B ready. While they may be fine for a while what is most likely to happen is that he starts mating them but they don’t know what is going on so they won’t cooperate. He is bigger than them so he forces them. It’s not pretty.
What I’d suggest since you really want him is to get him and keep him in a separate cage. After a couple of those pullets are laying, let them be together. There may be some excitement but they should be close enough to maturity to work it out. You can try putting them together now but be ready to go to this plan B.
