I do not think that there is one correct answer on this. We have some young dogs that started early, but until they were about 8 months old, I wanted to kill them every time I took them out because they never shut up. Now at 10 months, the extra mouth is almost gone. I think that it will be about right by the time they are adults.
On the other hand, I have seen some pups that started mouthy and as old dogs, they were still mouthy.