Average of 6ish months, but anywhere from 4 months to 9 months, depending on breed. You can tell by comb development, saddle and hackle feathers, crowing and, well, mating hens is a big sign
An older, dominant roo in a flock can inhibit younger roos from asserting themselves as males--less crowing, no mating, etc. And some breeds like cochins and brahmas are just naturally slower to mature.
Varies according to individual bird and breed. You can tell when he starts trying to mount your pullets/hens. He will start by grabbing their neck feathers with his beak and go from there. Once he gets the hang of it and the hens reach laying age, it is less aggressive. They will just squat for him.