All of my experience has been with St. Francis Service Dogs, which is the organization that trained Montana. Most of their dogs come into the program at eight weeks of age.
If the puppy or dog has problems and is taken out of the service dog program, there is a list of people waiting to adopt them. I know that Canine Companions for Independence (at least I think that's their name) also adopts out the dogs who don't make it through their program.
Some of these dogs are two years old and fully trained, but for some reason or other can't make it through the entire training program and level tests they have to take and pass in order to be matched with a candidate.
I don't know if this helps or not. Best of luck in finding out what you need to know.