Adult, egg laying age is approximately 20 weeks (really ranges between 17~18 weeks and 22~23 weeks), around 4 months generally speaking with some variety depending on different breeds. The boys, i.e. the roosters, tend to be overly eager and can start trying a few weeks earlier. At 3 weeks old they're 14 to 20 weeks away from breeding basically. If these Oj's are a difficult bird to breed they may have a later (i.e. longer) breeding cycle for example so they're fewer birds produced annually, often compounding the problem for species survival.
My mother grew up on a farm where they had all the typical livestock and she had a horse, along with the rest of the family, her two sisters and my grandparents. When I visited with my mother they still had most of the animals, and a LOT of chickens as they supplied most of the neighborhood with eggs still at that point. Sort of ironic as I think of it, I've got a lot of the actual knowledge with regards to farming that my character does, lol. A lot of general husbandry stuff, how to milk, and a lot about fruit and vegetables, especially fruit and nut bearing trees. You have never had good fruit until you've had fresh, ripe, off the tree / vine produce that hasn't had to be picked early for sale in a store! You're ruined for life after that, with regards to produce quality, heh. :D
Edit: Chickens, as chicks, are notoriously difficult to sex if you don't know what you're looking for. Even then a lot of people can guess wrong. It helps if you know the breed itself and what to look for in each. Its why I mentioned the wings and the layering. Most hens, even before displaying other gender based characteristics, grow their feather layers on their wings much earlier and visibly thicker than roosters do. So as long as they're young enough it can be the best way to tell.