When the hens let him! Often older hens just will not entertain a young rooster. But if he's strutting and crowing, he's most likely already trying to fertilise as well. Keep a close eye on them and watch for him mounting any hens. Or check for bullseyes in your eggs when you crack them open to eat them. If you know what you're looking for you can tell by looking at the surface of the yolk for either a little white dot (not fertilised) or a little white bullseye, like a target (fertilised).
