In response to this question, my usual response is “when do they start learning?”. They start learning the moment they are born and never stop. Mom usually does a great job with her pups because she of course speaks dog and so do they. At 8-10 weeks, they are usually removed from her and go to a family that does not speak dog. Even if they don’t train her, she is learning, and learning fast. A puppy will never be more “dog” than they are at this age. She just left the best pack leader she will ever had. One that didn’t feel bad about setting rules, boundaries, and limitations on every aspect of her life
Unfortunately, the more that dogs are around humans that do not understand them and speak their language, the worse it gets. In the first few months owners are content with the dog not going to the bathroom in the house or doing it less. We let them get away with things that their mother would not tolerate. The behaviors we allow because they are cute little puppies are the ones that cost us time, stress, and money as they mature. Things like nipping on skin, inappropriate chewing, and climbing all over us.
We are naturally designed to raise humans, not dogs. Human intuition and psychology do not work on them, you need to speak dog. Dogs speak with their movement and eyes, not their mouths. The more you talk the more the dog ignores because they see it as instability, like a dog that constantly barks.
It is a best practice to start training with working dogs such as service dog candidates at 8-10 weeks if possible. So with that in mind we like to start all dogs at that age when possible. Based on the over 2000 dogs we have trained, the vast majority of clients contact us when their dogs are 7-10 months and they just can’t put up with the bad behavior anymore. Initial training usually consists of teaching the dog what we would have started at 8-10 weeks. Point being that early intervention could save you months or even years of issues. Issues that not only cause problems and negative feelings between you and your dog, but also other family members and visitors.
So with all this said, the answer is it is best to start training as early as possible to give you the maximum time with a balanced and relaxed dog.