The experienced breeder I got chicks from last year advised me along the lines Chris suggests. If they are being raised in clean conditions it's hard to see why they would need worming before 18-20 weeks, since the usual time for treating with Flubenvet is every 6 months anyway, in a flock which is regularly wormed. It's a good idea to worm all your birds, including the pullets, before they are integrated at POL and then they are all worm-free and in synch. with each other for future treatments. But as Chuck and Chris say, do use Flubenvet because Vermex is a herbal tonic, not an effective wormer.