We've all heard the phrase you can't teach an old dog new tricks. Like many popular phrases, it's more catchy than true. Dogs live in the present, so the minute you start doing things differently, they will react. It is never too late or too early to start analyzing what is the best possible environment for your specific dog to be able to thrive. When I say training, I don't mean just teaching your dog not to pee in the house or how to do neat tricks on command, I mean communicating to your dog what is expected behavior and providing for his needs in a way that he can feel safe and fulfilled.
Mothers begin correcting their pups at a very young age. They learn social cues and rights and wrongs from their mother and their siblings. You ALWAYS want to take special precautions with every single dog you train. Consider exactly who the dog is and what his needs are. For instance, you don't want to neuter a male mastiff before the age of two because it can affect the way their hips grow. For this reason, you can have a lot of tension and drive in a very powerful dog, and if you're not breeding him, it is going to be more of a task to keep him under control than other dogs who can be safely fixed at a younger age. Again, you can always train a dog in the sense that I mentioned before. It is all about scientifically analyzing the best conditions for your dog no matter his circumstances.
Dogs are like children and many of them are great manipulators. They learn very quickly what they need to do in order to get you to do what they want. Because of their cuteness, it can be even easier for puppies to get away with things when they apply the proper manipulation techniques. These can include whining, barking, jumping, scratching, teething, eliminating in the home, chewing or anything that gets your attention. If you reinforce these behaviors with positive feedback, you're going to get more of them and it becomes dangerous when the dog gets bigger and older and has established his dominant position in the pack.