If you do a search there are other threads on this, but there doesn't appear to be a clear answer. It wouldn't hurt as far as I know, and if they haven't happened to get exposed yet then they'd certainly benefit. Someone else will have to weigh in on the guineas.
I think the day-old thing stems from the fact that most are done at the hatcheries and the benefit of doing it before outside exposure since many take their chicks out within the first week for field trips - we got a mixed flock with silkies so did our own at 2 weeks and they won't go outside until the 10-14 day period has passed for the vaccine to really start working. The main thing is to do it before exposure but who knows if your birds have been exposed or not. Some say it can still help. I guess it comes down to whether you're willing to pay the money to have it shipped and actually do the deed of injecting them. We paid about $60 total to get Marek's and Newcastle-Bronchitis vaccines and have them shippped from Jeffers in Alabama to us in CT. Worth it for us, not for everybody.
