It's best to wait until at least 18-20 weeks if you can, because any less than this and they will lack the confidence to stand up for themselves and can often be bullied by the older ones. Also, its good to keep them separate and feed growers pellets as long as possible, ideally until they are actually starting to lay, which may not be until 22-28 weeks, because until they have to make eggshell every day, the extra calcium in layers pellets isn't good for them. If you wait until 18-19 weeks, you can then worm them for the first time, and possibly do the rest of the flock at the same time, so they are all in the best of health and thereafter will need worming all together a few months later.