This is a trick question! I have started puppies when they were 3 mons, 4 mons, all the way to my best strike dog I have didn't start till he was 1-1/2 years old. I was ready to just about kill him and he struck and bayed his own hog. IMO,,I really think it is the dog that you are starting and how much he wants it.
x2!
Depends on the dog. The breeder of your pup should be able to give you the best insight on your pup. Especially if its line-bred.