How long until male ducks show the curl in their tail feather

[rw11279]

At what age (how many months old) do male ducks start showing the curl in their tail feathers, to where you can tell the males from the females?

 

[duckyfromoz]

Usually by three months old my drakes have at least a little curl developing- But by then it is very obvious from the voice alone which is male and which is female.