With the age difference in your flock, the very young birds and new hens brought in...I'd treat for coccidiosis asap even if just to rule it out. Sometimes the only symptoms you will see in adult birds are suddenly lethargic birds who die quickly. They can often fight it off longer then very young birds can but then suddenly be overwhelmed and just die. It would be high on my list of suspicions in this case. Not that there aren't a lot of other things it could be, it's just very common.
When you say these birds are down and can't get up, are they actually physically unable to move or get up or are they just too far gone to get up? Have you seen any of the splayed leg, one forward, one back position common to Merek's?
A necropsy by a poultry pathology lab is helpful because they do testing for just about everything you could think of. It's not just a matter of opening up the bird and having a lot, much more in depth then that.
Edited by cafarmgirl - 12/26/15 at 5:50pm