I have read that administering the vaccine can help with the survival rate of the chickens, at any age. Usually they recommend revaccinating also. It won't cure the mareks disease, but could keep the mortality lower.
There is much debate over this issue of vaccinating chickens that have been exposed and/or are older than day olds. It's worth a shot, as the turkey vaccine is not very expensive. It is only viable for one hour, once mixed though, so you have to vaccinate all the chickens, chicks included, in that time period, then discard the rest.
I understand it can only be bought in something like 1,000 dose lots. Seems a waste, but if it helps with even one chicken surviving this disease, IMHO, it is well worth it.
I would also recommend reading all you can on mareks. Not just for your own birds, but so that you are not, inadvertently, passing this disease to others. The transmition rate is tremendous and it is so easy to pass on to other chickens, that research is your best bet.
Another thing that seems to help some, is an oxine vapor spray...kinda like a vaporizer, to help them fight this disease...not sure how much it helps, but have read that some have had good reults with helping birds recover. If that is the route you are going to take. Most people will cull their entire flock and use the oxine to clean their land, as everything is now contaminated. There are no absolutes either, with mareks, with oxine cleaning the land...the only absolute is that now not only do your birds have it, you, in a way, have it as it is all over your land, your house, you, your car, your poultry houses, and on and on....it is mainly spread through the chickens dander and can travel for miles on the wind, live for years on the land and is a horrible disease.
If you are going to keep your mareks birds and try to get some to survive, if you do bring other birds onto your property, they must be vaccinated, preferably with the 3 vaccines available to the larger hatcheries. No birds should EVER leave your property and biosecurity is now a new term that you also will have to learn about.
I am so sorry you are going through this. There are a few threads on here that other OP's have started about mareks and is a great place to learn and for support through this. Best of luck to you and your birds and again, sooo very sorry you are having to go through this. If you search the top search box for maraks, look for the link that says " mareks in the flock, not an emergency" to get connected up with other OP's that are fighting this horrible horrible disease. Praying for you all!!!