There are currently 17 breeds where DM has been confirmed. And 124 breeds - which include both Norwich and Norfolk - where a mutant allele (the DM mutation) has been found that is highly correlated with DM when two copies are present.
The only evidence that Norwich do not get DM is that there has not been a CONFIRMED case. However DM is frequently mistaken as a spinal cord injury. So it is possible - and I suspect likely - that there has been a DM Norwich that has not been recognized as having DM.
The idea that Norwich might have timer genes that "turn on" the DM alleles only very late in life - as opposed to late middle age - is just a hypothesis. There is no evidence of which I am aware - other than no CONFIRMED DM Norwich - to back it up.
One can avoid the whole question of whether this hypothesis is correct or not by using the simple inexpensive DM DNA-test, and making sure that at least one parent of a breeding is DM-clear. This will result in no puppy having two copies of the DM mutation.
If one wants to breed two Norwich both of whom carry the DM-mutation, it seems to me that the responsible thing to do is to identify those puppies with two copies of the DM mutation using the simple inexpensive DM DNA-test, provide scientists with a sample of their DNA, carefully follow those dogs through out their lives, and make sure upon their death that their spinal cord is examined for DM. This would help those scientists who study DM.
The recent peer-reviewed scientific paper on DM () - of which Dr. Johnson of the University of Missouri is an co-author - recommends DM DNA-testing and avoiding producing dogs who have two copies of the DM mutation.
I urge everyone to know the DM-status of the dogs used in a breeding. If you do not know the DM-status, a simple inexpensive DNA-test can tell you the answer. And I urge everyone to share such results on the OFA web site.
 Breed Distribution of SOD1 Alleles Previously Associated with Canine Degenerative Myelopathy, Zeng et.al., Journal of Veterinary Internal Medicine, March 2014.