How young can you start training a puppy? I've often been asked at what age do you start training? The old thought was 6 months old; I honestly can't imagine waiting that long to start the ground work of training. But the reason behind the 6 months was to make sure that the puppies neck could take the collar corrections, makes me cringe even thinking about it. As far as I'm concerned as soon as you get your puppy, get at it.
Hopefully the breeder got a head start for you; many teach their puppies to sit and go to the bathroom on grass way before they are ready to go to their new homes. My daughters new girl who is now 7 1/2 weeks old and already has sit and down; and is ready to move on to bigger things. At this young age she is like a sponge and responds quickly to any feedback she receives.
Waiting for unwanted behaviors to form and then trying to get rid of them just doesn't make sense. So start as soon as that little puppy walks through your door. Not all training is to perform a position; lots of training is about instilling rules and regulations for living in your home; get off on the right foot. Don't mix messages; if you don't want your full grown dog on the couches, then don't let your puppy on the couches etc.
The other question I am asked often is how long do you train? Training should never be a session; it should be worked gently into your daily routine. This results in a dog who behaves all the time; not just during the training session. And push those little brains; typically people have no idea just how smart dogs are. I'm a trainer and they still blow me away with what they can learn.