One of my lil Silver roos started to crow at about 6 weeks, but it sounded more like a crow caw or a cough. He figured it all out, the day after he went to his new home at my chicken buddy's house. The next week, the other 2 started the same way. The pullets were always noticeably smaller and the cockerels started developing the wider combs, before that age, even. We were fairly certain of what we had by about 6 weeks old. In just a few days after the combs widened, I started seeing the "Leaders" developing at the rear of their combs. It was the easiest sexing that I have ever seen in chickens. I mean, it was soooo obvious, that I was surprised by it.
On the other hand, I also bought 5 (and received 6) SDW Phoenix Bantams, with that purchase (25 March hatch date) and have NOW decided that I have 4 surviving roos, out of 6 chicks. Just my luck as no one here has ever even heard of a Phoenix, let alone has a hen/pullet to sell. The lil cockerels started sprouting noticeable hackles, yesterday, tho I was already fairly certain, last week.
In my opinion, the 'Brights are easier to sex at 4-6 weeks than any other chicken that I know of, even tho the males are henny-feathered. Go figure?!