I lost all my olives to Marek's. They seem to be a lot less resistant to it than some of the other breeds. You can vaccinate them at any age. The issue is that you need to vaccinate them before they have been exposed to the virus. Experts say it takes three weeks (or longer) to develop immunity after the vaccine is given. it is recommended that the chicks be vaccinated as young as possible and then isolated for 3 - 6 weeks with no exposure to other chickens or bird dander. This means you have to wash well and change your clothes when feeding or handling the new birds. Eliminating all exposure is difficult if not impossible. I do the best I can and hope they don't get enough exposure to the virus before their immune systems can handle it.
@rich719
Do you have Marek's in your flock already? Have your chicks been exposed to the outdoors and your other chickens?
If the answer is yes, it won't do any good to vaccinate. If the answer is NO to both questions, you can vaccinate and hope the chicks have not been exposed already. It won't hurt to vaccinate them late, but it won't do any good if they are already carrying the virus.
Best of luck! There is also a marek's thread you can read as well as a Marek's Vaccine thread.

[/quote
Well yes theve been out there four about three weeks now but one question there were all raised together from cgicks by me so I dnt think theve been exposed to other chicken but ima try hope for the best