My birds are now about 6 weeks old. At what age will they start to develop so that I can tell which is is which?
I have RIW, NHR and BSL and my BSL are 5 weeks old and already have a good comb and waddles coming out. One of my RIW's is looking more like a roo and he's 7 weeks old. Usually the comb, waddle and eye area start growing considerably faster and turn the red or rose color much faster than the pullets. Granted we knew from the start that my BSL were cockrels. But my RIW suprised me. A good surprise though. He has always been one of my favorites even before we knew he was a "he". Don't know if this helps you or not. GL though.
Quick way is neck feathers. Males will have a pointed tip, females will be more rounded. Except for Sebrights, good luck wait for combs/wattles to differ.
Also in many breeds the males color in differently. May I suggest this great site: http://www.feathersite.com/
I have some 3 week old chicks and I can already tell with three of them that they are roosters. Much larger combs and they are already starting to get slight pinkish reddish at base of comb.