Speckles and Freckles at about 5 weeks or so. (Granddaughter named them - Teehee!) Even at this age the differences were easy to see. Speckles was a cockerel, Freckles a pullet.
Same positions - Speckles left, who spotted a fly, Freckles right. I think they were about 5 - 6 months old here. Of all the chickens I have, they were the easiest to tell apart, even early on. Freckles carried a lot of green in her wings and near her tail.