drsophiayin.com/blog/entry/teaching_rover_to_race_to_you_on_cue
As soon as your puppy has received their first set of vaccinations. The most important stage for socialization is the first 3 months of your puppy's life. During this time puppies should be exposed to as many new people, animals, stimuli and environments as can be achieved safely and without causing over stimulation, excessive fear, withdrawal or avoidance.
For more information please read the following statement published by the American Veterinary Society of Animal Behavior.
www.avsabonline.org/avsabonline/images/stories/Position_Statements/puppy%20socialization.pdf