There is some controversy surrounding the test, specifically it is not 100% clear that the genetic marker they have identified for DM is the actual marker in GSDs as some dogs have had a diagnosis of DM on necropsy (the only definitive way to diagnose the disease) who have not had the marker or only had one copy (it is recessive and allegedly needs two markers to express itself). Univ of Missouri as well as U of Florida were doing research on the disease and came up with different theories. U of M got the money and the U of F program slowed way down.
I would guess since they are looking at DNA age would not matter.
Orthopedic Foundation for Animals
Degenerative Myelopathy - Using the DNA test