It depends on the age of the dog, the breed and behaviour of the dog. Usually, a puppy takes less time to learn than an adult dog.
It is as early as possible. You can train your puppy. It is the best time or an ideal age to train a dog.
Yes, it is difficult to train a matured dog. It can take more time to learn things than a puppy.
Yes, it is. However, it is treatable with good behavioural training.