My BO's are 18 weeks old. I ordered them from Purely Poultry un-vaccinated. They are the only chickens I've ever had and I don't plan on adding any more into the flock. I've read a bunch of posts on Mareks and it seems like the older the chickens are, the less effective the vaccine is. So what is the general opinion on vaccinating 'older' birds? I'm leaning towards vaccinating them since they're not out of the danger zone for the usual age that they can catch the virus. Thanks!