In my (short) experience with Barred Rocks, I've discovered it depends on the chick and their rate of development. Not sure about other breeds. One of my cockerels, Zeus, was an obvious boy...his wattles and comb grew fast, huge, and red. Pixie...now I thought HE was a SHE...because it took him...
The only ABSOLUTE way to be sure is when the hens start laying eggs and the cockerels start crowing LOL
But, you can be almost absolutely sure between a month and three months, depending on the breed of chicken.
Would it help to take pictures? YES!
If you have chicks with huge wattles and...