I've had them crow at five days old (yep, days, not weeks). They can crow any time. Most of mine crow between 8-15 weeks old. They usually become fertile between 16-18 weeks old, but may have been mating and crowing long before that.
My senior rooster raptor, sounded like he was gagging until recently. His efforts at practicing has paid off and his crow is beautiful! He's about 16 weeks. The other rooster seems to have no voice and the third one just suddenly started crowing last week. They are all the same age. GL
One of my roos started crowing at 4 weeks! I couldn't believe it and then he did it again with me watching. They ar about 8 weeks old and haven't tried to start mating yet. Thank goodness because I am not really looking forward to explaining that behavior to my 5 year old.