At their age I would wager that they are testing the waters and working on pecking order within their group. My cockerels from a June hatch are just now starting to spar with one another even as they learn to crow. It's so cute when they are little. I just had two 9 month old half brothers go at it last week. I expected the larger of the two to come out on top but he wound up with a muscle injury in his hip. They do mean it when they are mature and fight.
Your roosters will start to show interest in the hens before the hens start to lay. They mature faster. When you notice them trying to grab the hens by the back of their neck feathers and mount them, then it is time to either remove all the cockerels or leave those in that you want to use for your breeding program. They won't have any finesse. The hens will scream, feathers will fly at first but they will learn as they mature.