How old can a puppy be trained?

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.