I'm sorry but, there are many of your posts that I would like to shed a little light on. First, let me give you a little background about myself. In the past year I have been fighting Marek's in my flock. It was diagnosed through necropsy on two different birds, two totally different sets of symptoms. I did this to make sure that I wasn't dealing with two or more different diseases. To date, I have lost 35+ birds. I feel, at this point, that I am an expert on Marek's. Self proclaimed, yes...but, at this point there is nothing that I haven't seen. I kept one pullet alive for over 10 months...I watched what Marek's could and does do. Some will not like the fact that I used her for a guinea pig. Let me point out. She was a vaccinated OEGB pullet. Therefore, she didn't get the normal tumors that kill. She was never in any pain or discomfort. She lived in my kitchen surrounded by those that loved her dearly, played with her and talked to her all throughout the day. Enough of that, it just about killed me when she died. But, she gave me so much information about this disease. Information that you can't find in books.
Now, I would love to know where you found your information. Mine reads entirely different. One can expect to lose up to 80% of a flock to Marek's. I do not consider that a low to moderate rate of loss.
I have had birds both vaccinated and unvaccinated die. On the flip side, I have had some from both sets live. Out of my original 37 birds, in my coop, I have 7 left. That is just from the coop. I have lost many more that have never made it to the coop.
As far as the Merks manual goes...I say that you can keep it. There is only one thing in there that at this point I will agree with. Marek's is transient. Meaning that it can go into remission and stay there for awhile...but, man when it comes back, watch out. I have a black bantam cochin hen that has been in remission for over a month...laying eggs like crazy. Now, just like a flip of a switch, she is down and can't use her legs. What happened? Something very simple...I put her outside in a cage for some sun. That's all it took...it must have been a little stressful and Marek's once again reared its ugly head.
As for ages..I've lost chicks from 10 weeks to chickens at 15 months old. There really is no age barrier or limit. Marek's can and will kill any age chicken. Where that age thing came from, I will never know. It certainly wasn't from a good field study.
The lowest rate of mortality that I have had has come from chicks that I hatched from my own eggs. They were vaccinated at day 1 and keep in isolation for two weeks. I have only lost one of those out of 17. The vaccinated pullets that I purchased this spring haven't been so lucky. They were kept in isolation for 14 weeks and still, I have lost 4 out of 9. Remember, the vaccine only stops the formation of tumors, not the paralysis and other nasty things that Marek's does.
I have so much info in my head that I think that I will stop now. Feel free to ask any questions that you might have.
I have lost all but 1 of my polish to Marek's. Marek's seems to favor silkies and polish. Those are the breeds that seem to be hit the hardest.
If there is a disease that is worse then Marek's out there, please don't send it my way...since, Marek's is the longest lived virus and stays around the longest, there is no getting rid of it unless you want to euthanize everything and wait 7 years to start again.
Mareks typically has a low to moderate mortality rate compared to other diseases, and it isn't the worst out there.
I've dealt with mareks and IB being introduced into my flock by someone dumping a sick bird in my yard years ago...I had to cull all the birds on the property we had at the time to not risk spreading the disease(s) to others. 
