The “critical period of socialization” for a puppy is between 3 and 13 weeks of age. It starts when the puppy opens her eyes at 3 weeks of age and is strongest for the next 10 weeks, but dogs never lose the ability to bond.
From 3 to 5 weeks old, puppies explore everything new in their environment and show no fear. By 5 weeks of age, they start avoiding new things and situations.
Puppies are usually separated from their litter mates and placed with people between 6 and 8 weeks of age but it’s best if they get a lot of social interaction with people starting a three weeks. Puppies that are removed from their litters earlier often don’t know how to interact with other dogs! (Many of us may have know one of these dogs – does your dog think it’s human?)
Information from Dog Behavior: An Owner’s Guide to a Happy Healthy Pet.