I think it depends on the breed. Our Golden Campine first crowed at 5 weeks and our Single Comb Brown Leghorn didn't crow until he was 10 weeks old.
My RIR roo started crowing at about 10 weeks...Couln't call it much of a crow, more like a croak. At 15 weeks he's got the hang of it, and he's chasing the ladies around the yard.
All of my EE roos started right at 8 weeks. My copper black marans roo was 10 weeks. Ameraucana roo crowed when he was about 4 weeks!
I had one at 6 weeks, one just started at 11.5 weeks, and I have two 22 week olds that haven't made a peep...