Usually at 1 year of age, but some became fertile at 2 years. Yes, it's quite a long time and I also can't wait for my chickens to have chicks, but patience is the key. Although the roo will be interested in mating at 5 months old, the eggs will not be fertilized.
I have RIR's and the roo became fertile before the hens started laying,and that was about 20 weeks, but he loves them too much now, once i get over this drated flu i am gonna have to move him to a spot all alone to give my ladies a rest, and regrow the feathers on their backs,
Owned by 12 RIR.1 Pitt bull, 1 poodle mix,1 yorkie,1great Dane ,1 crabby gray cat and most important 1 adorable DH
Roosters generally mature at the same rate as the pullets of the same breed. It's generally not "are they producing sperm" more "are they able to successfully mate the pullet"
It takes practice for a roo to complete the job, they are often awkward and fall off a lot at first.