American Veterinary Society of Animal Behavior

"The primary and most important time for puppy socialization is the first three months of 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 manifested as  excessive fear, withdrawal or avoidance behavior".

http://avsabonline.org/uploads/position_statements/Puppy_Socialization_Position_download_-_10-4-14.pdf