It is just so hard to tell at this age. I think my last bunch last fall I kept looking at two of them and one day thinking it was a pullet, next day cockerel. I don't remember exactly, but it seems like they were about 8 weeks before I was 90% sure, and then another 4 weeks before I was 100% sure. I'll have to look back at my pictures to see. I do remember that there was a significant difference in feathering at about 8 weeks though. The pullet had her chest feathers and the cockerel didn't have very many.
I've got the same problem right now with the 3 that are almost 5 weeks. Some days I think they are all the same sex, some days not, some days I have 2 pullets, some days 1 and some days none. It is really hard to be patient!