The earlier the better, but six months or ten years old does not mean the dog can not be trained. I would say again though, go to training together, don't send the dog on it's own. This method has it's merits, but only when you have a great relationship with the trainer and you have very good knowledge of dog psychology and training and are confident that you know how to carry on the process - as said before it is an ongoing thing - and don't we all know it takes a second to undo a month and half of training?! Please go to a puppy class with the dog and both start to learn - you will never regret it, I promise!