I have no idea if he's fertile yet, but one of my roos started crowing at 8 weeks and trying to mount my pullets at 10 weeks. Since the girls are the same age as him, they are not laying yet (now 12.5 weeks old), so if he's fertile, it's going to waste... but he's sure acting like it... maybe it's just practice? This is an interesting question and I would love to hear the answer if anyone knows. I know I've read that it's best to wait until your hens and roos are at least 1 year old before you allow them to "become parents" - ie you should hatch eggs from a roo that is 1 year or older, and you shouldn't let hens hatch until they are at least a year old... but maybe that's not even true. I've only read it, but I have no real idea.