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Puppy Toddlers (3 - 6 Weeks) During the Toddler period, puppies emerge on their own from the litter. They venture into the surrounding environment. This emergence from the litter is a gradual and continual learning experience. During this stage of development puppies learn basic behavioral patterns specific to dogs. While playing, they practice different body postures, learning what the postures mean and how they affect their mother and litter mates. They learn what it is like to bite and be bitten, what barking and other vocalizations mean and how to make and use them to establish social relationships with other dogs. Such learning and activity tempers their own biting and vocalizing. From the age of five weeks, the mother teaches her puppies basic manners. They learn to be submissive to her leadership and what behaviors are acceptable. If necessary, she growls, snarls, or snaps at them as a form of discipline. When weaning the litter, for instance, the mother will discipline her puppies so that they will leave her alone. Because the mother disciplines them in a way that they clearly understand, after a few repetitions, the puppies will respond to a mere glare from her. If a pup has not learned to accept leadership (and discipline) in its early interactions with dogs, its training will be more difficult. Puppies that are removed from the nest too early tend to be nervous, more prone to barking and biting, and less responsive to discipline. Often they are aggressive with other dogs. Generally speaking, a puppy taken away from it's mother and litter mates before seven weeks of age, may not realize its full potential as a dog and companion. To maximize the mental and psychological development of puppies, they must remain in the nest with their mother and litter mates until seven weeks of age.

Socialization Period (7 - 12 Weeks) It is at this age that rapid learning occurs. At seven weeks, puppies can learn and what they learn will have a lasting impact. Everything he comes in contact with will make a lasting impression upon him as it never will again. Not only will he learn, but, he will learn whether he is taught or not. Though he has a short attention span, what things he learns are learned permanently and resistant to change. Therefore, owners need to be careful about what their puppies are learning at this time. Your puppy is very anxious to learn how you want him to behave and react, and he needs to be shown what is expected of him in his new role as your pet. There are rules you will expect your puppy to obey. Establish those rules NOW while behaviors are easy to establish. For instance, how your pet interacts with you is determined during puppyhood. What he does now is what he will likely do later. So, don't allow your puppy to do things which will be unacceptable when he becomes a dog. During this time, you and your puppy will also begin to know and understand each other. You will get to know about your puppy's particular temperament and personality - whether he is strong-willed or eager to please, gentle or rambunctious, shy or outgoing, and just what else makes him the endearing individual that he is. For the puppy, this is both an exciting and somewhat confusing time. There is a whole new world of things to learn about and all sorts of new experiences to digest. Remember that the environments you put your puppy in are more complex than those he would encounter naturally. Puppies must now learn a new set of rules. He needs to know learn how to interact with humans and other animals who live with them. Puppies must adapt to the patterns and tenor of their new homes. All of these experiences and the behaviors which accompany them, must be learned. Because you will impose such important demands on your puppy, you must help him to make the transition into the human environment. You need to lay a groundwork for a trusting, happy mutually satisfying relationship. Keep in mind that puppies are less likely to broaden their experiences if they are insecure. In natural environments, puppies approach new things cautiously. By giving your puppy brief, repeated experiences in new situations, you give him a chance to become familiar. If you don't expose your puppy to a variety of situations and new environments, inappropriate ways to adapt may be learned. During the Socialization period, there is a fear imprint period from 8 - 11 weeks. During this time, any traumatic, painful or frightening experiences will have a more lasting impact on your pup than they would if they occurred at any other time. An unpleasant trip to the veterinarian, for instance, at this time could forever make your dog apprehensive about vets. To avoid this, take some treats and a toy with you. While you wait, play with your puppy and offer him treats. Have your vet give your puppy treats along with lots of praise and petting before and after the examination. Avoid elective surgeries, such as ear-cropping and hernia repair during this time. In general, avoid stressful situations. Remember, dogs are social animals. To become acceptable companions, they need to interact with you, your family, and other people and dogs during the Socialization Period. Dogs that are denied socialization during this critical period often become unpredictable because they are fearful or aggressive. It is during this time, that your dog needs to have positive experiences with people and dogs. Therefore, you need to socialize and teach your puppy how to interact with people and other dogs in a positive, non-punitive manner. You should gradually introduce your puppy to new things, environments, and people. But, care must be taken in socializing your puppy with other dogs or in areas where many "unknown" dogs frequent, prior to the time that your dog has had three of its booster vaccinations against contagious diseases. Shopping centers, parks, and playgrounds are good places to expose him. Begin by taking your puppy when there are few distracters. Give him time to get used to new places. Make sure he is secure. If you have children that visit only occasionally, have your puppy meet children as often as you can. If you live alone, make an effort to have friends visit you, especially members of the opposite sex so that your dog will become accustomed to them. If you plan on taking your dog to dog shows or using your dog in a breeding program, get him around other dogs. If you plan to travel with your dog, get him accustomed to riding in the car. Take him for brief rides, at first. Go someplace fun. Remember, if new experiences are overwhelming or negative, the results could be traumatic.

Seniority Classification Period (12-16 Weeks) It is during this critical period that your dog will begin to test you to see who the pack leader is going to be. He'll begin to bite you, in play or as a real challenge to your authority. Such behavior is natural in the pack and not necessarily undesirable. What is undesirable is an inappropriate response on your part. It is important, at this stage, that you establish your position as pack leader, and not just another sibling. Other behaviors, such as grabbing at the leash, will be observed, and all are attempts to dominate you. Biting , in particular though, should always be discouraged. Therefore, you should not wrestle or play tug of war. Such play is aggressive-inducing. What you see as a fun game may be perceived by your dog as a situation in which he has been allowed to dominate. Wrestling, of course, communicates to your puppy that he is allowed to bite you. Tug of war sets you up in a dominance confrontation over an object. He learns that he can keep objects away from you. During tug of war games, puppies will often growl. Growling is a dominance vocalization, designed to warn another pack member that they better not confront the growler or he will bite. Puppies see these games as situations in which they have been allowed to dominate. They do not understand that these are games designed by humans to entertain them. You can continue to play with your dog during this period, but, the relationship between you during the play must change. No mouthing of your body should be allowed and when your dog does mouthe, you should respond with a quick and sharp "NO!" or "No Bite!" Play that does not get rough is best. If you cannot keep the dog from getting overly excited during a game and he persists in biting at you, don't play that way. This will only stimulate additional dominant behavior in the future. For these reasons, this is the stage when serious training should begin. Training establishes your pack leadership in a manner that your puppy will understand. By training your puppy, you will learn how to get him to respond to commands designed to show that you are in charge.

the bold in Socialization Period (7 - 12 Weeks) above-is one of the reasons 12 weeks is a good time.

I put most of the article in for the folks who wanted a little more info on their puppies development.