They are currently in the big cooler, filled with salty ice water. I remember reading that that was a good way to rest them. The 7 girls seem so much happier now that there aren't 11 boys (9 leghorns and 2 red broilers) jumping on them every other second. =)
So we should make sure to do the girls before they turn 16 weeks old?
What breed are the females? Red Broilers? If Red Broilers are similar to Red Rangers (my only experience in a colored broiler), then they should be already be at butchering size by 14-16 weeks. But hens will stay tender longer than roosters, so they'd probably be fine on into 18-20 weeks if you want a very big bird. I wouldn't go any later than point of lay, though.