I have 3 chicks 9 weeks old , I was wondering how to tell if any are roosters and the only way I know is when they start to crow. Is there a certain age that they start to crow?
Depending on the breed it can be between 5 weeks and 20 weeks. If the birds are all the same breed, cockerels will be a bit taller with thicker legs, bigger combs and wattles. If they're different breeds it makes it more difficult because some breeds of pullets have bigger combs than other breeds of cockerels. Eventually cockerels develop pointy hackle and wing feathers and curved tail feathers.
9 weeks you should be able to tell. Post some good close up pictures that show the whole body in particular the neck and saddle feathers and people can probably help you.