My new chicks (got them last march) started laying eggs in july ......so guess as long as the roo is mature and the hens are laying , you can get chicks at anytime . Some people have hens who take a long time to start laying though !
They can lay fertile eggs from the very first egg, however most do not recommend hatching the first eggs let the hen get her system worked out first. If there is a roo in with them, when they start laying chances are its fertile.