My girls are 10 months old and getting their first worming today. My BR has roundworms quite badly, and is looking very droopy. I didn't know she had worms. If she does, everyone else probably does, so the entire flock is getting wormed.
From what I've read, chickens should be wormed when they move to the coop or around 4-6 months.
2. WITHDRAWL PERIOD
This really depends upon what you use. Various dewormers have widely different withdrawal periods, from 0 to up to 17 days. Google will help you decide what's best.