Maybe 3-4 months, the hens have a well put together crest, as for the roosters it's spikey and dangling ( usually ), also at some point you will see the difference in the tail feathers this could be at 5-6 month i'm not sure, because i raised polish and they are now 10 months old and i can clearly tell the difference but when i first started i didn't know anything and i would just pick things up along the way, if i raise polish chicks again i will certainly pay more attention to the details and be able to tell them apart easily, also at 3-4 months if you really pay attention you can see that the roosters are more hyper, curious and outgoing, as for the females they're shy not very hyper, and can be seen following the hyper ones ( probably roosters ) around. Finally, when they reached 7 months old my father told me that a long time ago when he had chicken, the roosters would have an extra small toe, above the foot and at rhe back of the leg, so i checked it and he was right i had two with extra toes which i had already identified them as rooster at an earlier age, and two hens with no toes so this way i was 100%sure of their sex. ( note that my SILKIE roosters don't have extra toes ) so maybe it's just for polish. Again i'm no expert but these are the things i picked up and helped me identify the roosters from the hens.
.. my black crested white polish, but i wonder if there are going to be that
Your photo shows quite clearly why a polish crest will attract pecking. The feather bases are still enclosed in the feather sheaths, which is normal because a polish can't preen it's own head to remove them.
It is definitely not a white crested. Almost looks more like an appenzellar.
Looking rooster to me.
