Preferably at 7-8 weeks old when it is the optimum time to bring a puppy home. At this time we do not teach formal obedience, we teach manners and constuctive play, in order to imprint good habits now rather than trying to eliminate bad ones later on. Formal obedience can begin at 6 months of age and older. As long as the dog doesn't have any medical reasons preventing him from training - than age is not a problem. This varies from owner to owner.
Many people like to go on to off leash obedience, but it depends on the individual owner and dog.
It's not natural to train our dogs. We should leave them as they are. First let me say that I'm glad you're not my neighbour. Then let me say that you have obviously never seen a dog get hit by a car, choke on a chicken bone, or attack a child. Dog ownership is a priveledge, and with the priveledge comes a great responsability.