At about 10 months of age you can be pretty sure, but there are a few animals that might look like a female at 10 but turn out to be male. Generally at 10 months you should have a pretty good idea. If you do decide to breed them you still need to wait until they are about 14 months old. Even if you know your animal is female at 10 months it is still too small. Some people say that their animals are big enough at 12, but it is a good idea to wait. If you don't the female could have complications with laying her eggs and die. So it's better to be safe than sorry.
:i have a young crested right now and i am considering breeding in the future. once i sex mine than i could purchase a mate of the opposite sex.