At what age can you tell the gender of these birds?

[DolceVita]

At what age can you tell the gender of Orpingtons, Cochins and Brahmas and silkies

 

[Gray Farms]

6/7 weeks for Orpington and Cochin, less depending on pattern. 8/9 weeks for Brahmas, less depening on pattern. 3/4 months for Silkies.