My experiences pretty much mirror Centrarchid's. Each chicken, male and female, is an individual. I once had a cockerel about 5 months old capable of dominating older hens and basically being the flock master. I also once had a cockerel that could not manage that until he was 11 months old. The 11 month old got the job done but was not my best rooster. On average I'd say 7 months is a good number but it can vary. Typically by 5 months the cockerels are chasing and mating pullets their own age (usually by force) and maybe a few less dominant hens. This can look quite violent. The 7 month number is when mine typically have won over all the mature hens and it calms down a lot.
What people tend to ignore is that females are individuals too. When you have a flock of hens with no dominant male the dominant hen often takes on many of the roles and responsibilities of a dominant rooster. Some dominant hens are more willing than others to give up the role of being boss. That 11 month old, for example. By 7 month she was fertilizing some pullets and less dominant hens but the dominant hen would knock him off if she saw him to let him know she was boss. When he finally matured enough to stand up to her (he was a lot bigger) they fought for two days before she finally gave up and submitted to him. Some hens seem to be glad to give up those responsibilities.
Thee are always exceptions to anything we call normal in chicken behaviors, it just works that way with individuals and their different personalities. How we manage them can make a difference too, whether they totally free range or we keep them confined in a pen of some size. But unless you have some really strange chickens at some point he will fertilize the eggs and take on the other roles of flock master. I just can't tell you a specific age because I've seen it vary so much.
