Immunity is a weird thing... so unpredictable!If it was Marek's I'd except the younger birds to have succumbed by now wouldn't you?
Molt is hell for some.
Since it seems your older group who is experiencing the most issue, I would take a double look make sure no toxins are lurking.
Do you already have this symptom checker tool...
http://www.poultrydvm.com/views/symptoms.php
You put a symptom and it tell you the many possibilities. What I like the most is when I am using it... kinda reminds me or informs me of other symptoms to looks out for or learn to notice. It just gives you leads for further research, but still a useful tool.
Sorry to hear you have yet more cause for concern... it won't be all bad forever!