How old can a puppy be trained?

I know it is easier to train dogs when they are younger, but it is obvious you can't train them upon birth.
Just how old a puppy should be when you first start to train it for the best results?

You can start training pups as early as six weeks old.Just remember to use positive reinforcements when you are training your puppy never use negative punishments if the pup does not learn immediately what you are trying to teach him/her.

Six weeks old is a lot earlier than I expected. I can confirm that positive reinforcements are better than negative punishments. First hand experience from how my parents "trained" me. :p In fact, negative punishments never work.

When you talk about positive reinforcements for a young puppy, I assume it is a small treat right?

Positive reinforcement, can be either a small treat, high praise or a good fuss, so some thing as simple as a rub on the head and lots of good boy would suffice.
some breeders will start the toilet training while the pups are still in there care, you should start training your pup as soon as you get him home.

Train the puppy from the minute it can leave its mum. The earlier you start training the better for the pup and you to get a bond together.

I think the best choice would be after a couple of months they are born. If you try to train them immediately after birth, you would have some difficulties. Firstly, they are not familiar with their environment and they're scared most of the time. All they want to do is be held, fed and loved. So, once their eyes are open and they're able to walk and run well on their own then I think that's the best time to start.