How old do chicks need to be before sexing?

[Kiwi Chicky]

We have 5 3week olds and 10 2week olds and we'd like to know what sex they are!

How old before we can try?
Any hints as to what to look for?

They're barred rock crosses and pure breed light sussex.

 

[DuckFanatic]

IMO, 2 to 3 week olds are still pretty early to tell the sex in most chicks. However, in barred rocks it can be easier to tell because, once they start feathering in, the best way to tell is that the roos will have more white on them and be lighter and the pullets will be darker with less white.... not sure about the light sussexes though