As far as I know, chickens of all ages are susceptible.....i think its just more prevalent in the younger ones. Do yours have blood in their poo? You can put them on a low dose corid from the co-op. The only kind I have been able to find is for calfs but its the same stuff....you just have to give them a much smaller dose. If the have respiratory problems though you will need to make sure they are warm in order to get them well and give them either sulmet or terramycin. All can be found at co-op or a local feed store. Do you have any idea what their symptoms are? If you get down real close and you can hear a clicking or rasping noise while they are breathing its probably respiratory related.