Well, I don't know. For whatever reason, I never thought about that. So I just researched it a little. It was kind of hard to find a decent answer. The answer I did find should not be taken as absolute though because as far as I know, the person who wrote it is not an avian vet or vet of any kind, actually. But here is what he said:
The myth is that you can only vaccinate day old chicks. You can actually vaccinate your birds at any age. The difference is that as every day goes by, you run a greater risk of your birds being exposed to the virus before you have a chance to vaccinate. Two more myths are that you can’t mix vaccinated and unvaccinated chicks together and that you can give your flock Mareks by exposing them to vaccinated chicks. Since the Mareks vaccine is actually the Turkey virus, you cannot give your birds Mareks by vaccinating. Also, vaccinated chicks cannot hurt unvaccinated chicks by mixing them together. My personal practice is to ‘booster’ my adults every year when I vaccinate my chicks – day old or otherwise.
