Breeders will be allowing young pups to experience new things from 3 weeks until they are handed over to their new owners at approx 7/8 weeks old. New owners need to carry on introducing new things to the puppy and it becomes the owners duty and responsibility to cary on socializing and habituating the puppy to things.
The difference between socialisation and habituation is that socialisation allows the puppy to learn canine etiquette, read their own species body language and engage in correct and accepted behaviour amongst other dogs, all of which are learning skills that help a young puppy develop as he grows. Starting socialisation at this early age prevents possible future behavioural problems and teaches him about other dogs intentions, and how he should and should not react to them.
Habituation allows a dog to become accustomed to things. This may be sights and sounds on a walk, the hoover, other dogs, traffic, anything that he encounters! The more times he is exposed to things in the world around him, the less likely he will be to react negatively.
Habituation can be temporary, owners need to be consistent and constant in their dogs exposure to things, for example, a dog who has not been out for a while, perhaps due to a long illness, will need to be introduced to traffic again slowly and in short durations. If done too fast and intensely or when there’s too much traffic to cope with, he could have an aversive reaction to it, causing future behaviour problems. After 16 weeks of age it becomes much more difficult to introduce your puppy to new things without fears, phobias and antisocial behaviour becoming a problem.