Personally I like to put colored leg bands on the birds well before it's time to choose who stays and who goes. That way I can watch them as they grow and note the ones that grow faster/slower, who starts laying first, who starts crowing first, who is a jackass and will be eaten, who is more dominant/subordinate. When you're running birds that all look the same, one of them can catch your eye for something good or bad, but they get back into the group and you can't tell one from the other unless they have something to ID them.
I like to separate by gender as soon as I can tell who is who and housing is ready for them. So far separating them by at least 2 months old is working better for us. Get them separated well before anybody gets an idea that there is such a thing as the opposite sex. It also has helped being able to put in two different age groups together easier because none of the males were anywhere near maturity yet and they were all still young and scared to be thrown out into the pen so I didn't have to build a creep feeder for the younger ones.