It depends on the breed, some are slower maturing than others (big breeds: Brahmas for instance, or Jerseys) but most standard sized breeds start showing interest and working on it by 4-5 months (also crowing a lot, doing little dances for the hens, etc., very cute). Some are precocious and get into it a little earlier...
It also depends upon whether you have other roos around. If there is a dominant male, the young guns won't mate early (or at least not whilst alpha male is around!).
I currently have 25 week old Brahma boys who show no sign of mating as I have several other adult males. My 28 week old NN X Brahma mates with any and everything as he is such a fast runner, but he only ever crows when he is a very long distance from the 'king pins'.