My bantams seem to crow earlier than any large fowl rooster I've had. My silver Sebright cockerel crowed for the first time at 6 weeks, while my 3 other Sebright cockerels haven't even attempted to crow yet. My EE rooster crowed for the first time at 9 months at about 9:30 at night. It was pretty surprising since I thought I had all hens... And my BR rooster is 4 months and crowing.
I've also heard that sometimes if you have multiple roosters, the beta rooster won't crow to keep under the radar of the alpha. My sebrights are now 4 months and I haven't heard a crow out of the rest if the males so that could be the case. Or they haven't found their crow yet