oh dear
I can speak from experience of Mareks disease
when I first got chooks I had 3 from a friend - they were 18 months old when the first one had symptoms , we had her PTS , then 3 months later the second one got it and had to be PTS
That left me with 1 - at the time I was new to keeping chooks and didn't know about mareks and how it can spread
we decided to get 3 new 12 week old hens needless to say they all died within 6 weeks
after speaking with the vet he said the reason the 1 hen from the original hens had not contracted it was because her immune system had managed to be strong enough to survive it but she would now be a carrier of the disease for life ( incidentally she lived for 7 more years )
After that we have always had fully vaccinated birds and for the last 10 years have not seen the disease return
so back to your birds , if the old ones are fully vaccinated they hopefully will be ok but if not there is every chance it will spread , younger birds are more susceptible to it but as I experienced you can loose older birds too
I would advise trying to keep the older birds separate it may help but I fear you may possibly loose some of them , once you've had it , it can stay in the soil and surroundings for up to 8 months so even quarantine will not guarantee preventing a reinfection
this is the breeder I buy from - and her page on disease is quite helpful
http://www.wylyevalleychickens.co.uk/diseases/#MareksI wish you luck it is an awful disease to experience