Honestly, it sounds like CRD... probably an MG infection.
Your older birds are probably doing well because they have had it, are carriers, and are shedding enough of the virus that the younger ones are now getting. It's most deadly to chicks usually-- might explain why the chicks you got before died easily.
Birds can "get over it", however, they remain carriers and can shed the "virus" at different times. (I say "virus", but it's actually a bacteria that acts like a virus) They can get symptoms again when stressed/cold/etc. They CAN live with it just fine, some/many may never show symptoms again but be able to infect new birds. It can be passed on to hatching eggs from a carrier hen. Not always, but it can... usually during a time when there is more shedding of the 'virus' going on, or during an acute illness of it.
Many people will cull/depopulate because of this carrier status. They can no longer sell chicks, hatching eggs, birds without infecting other flocks.
Many people will treat any secondary infections from an outbreak, keep a closed flock (no in, no out), and 'live with it'.
Some people will keep a closed flock but choose not to treat any 'flare ups' and let only those that can kick the illness themselves each time survive.
It only lives about 3 days, tops, outside of it's host.
The incubation is about 6-21 days... it's slow moving, and not all will come down with symptoms, but can still be infected or carriers. Not all may be infected/carriers at all.
Typical symptoms are: Watery or bubbly eyes, sneezing, coughing, raspy breathing, sometimes heard mostly during roosting at night. Nostril discharge, usually clear or thinner. Some listlessness/depression.
It's estimated that a large number of backyard flocks are infected with this at some point-- many completely unaware. Most hatcheries don't test for it, and may get their eggs from MG positive flocks. Wild birds can carry it/transfer it.
It's frustrating and overwhelming, for sure!
Your options are complete depopulation, sanitizing, waiting some time, and get birds only from clean stock, preferably hatching eggs...or day old chicks.
Treat your flock/live with it...and do not let any bird leave your property to possibly spread the illness. Or hatching eggs. Any new birds brought in will likely be infected and become carriers if they get over the initial infection.
Sulmet doens't do much for it, I don't think...
Tylan 50 (marketed for cows/pigs) or baytril (only by Rx from vet) are the best if you chose to treat it and close your flock.
Tylan comes in injectable (but can be given by oral dose) or in a formula to mix in the water---but that seems slightly less effective.
It can be injected, of course, and this is likely the best way to get the med to work the fastest...but some people don't like to inject them or aren't comfortable doing it.
Per my vet, you can mix several meds (water soluble) also... aeromycin+tylan soluble AND even give a Tylan injection... I'd probably start much less severe unless you have a very, very sick bird you must save.
Look in the Livestock section for Tylan 50... in the cow/pig area. Places like Atwood,
TSC, and some feed stores may carry it. It can be bought online, too.
I'd say decide if it's cull them all...or take the 'live with it and manage it' approach. Kinda depends, possibly, on what your goals were for your flock. If you wanted to buy/sell/hatch or sell hatching eggs to others--- this should NOT be done if you decide to treat and live with it.
MUCH, MUCH care must be taken if you choose to cull and start again--- it's so common and "out there", it's too easy to infect another flock...
