How old do baby chicks need to be before you can start to worm them? Have 8 chicks that are going on 5 weeks old. Also concerns about integrating new chicks with old hens. Maybe the light is the solution. Thought the idea was to keep the old hens from noticing the new chicks being mixed in? My chicks have all their feathers by now so not too concerned about the cooler weather. We live in the South so we are not going to get real cold weather for awhile....probably December into January, February, March then start warming up again.