Hi Maryellen!
I believe that one of the first fear "imprint" stages begin as early as 6 weeks (with two particular stages during this age to about 12 weeks) when the puppies feel the urge to explore beyond their known area, venture away from mom and littermates and that biological "self-preservation" instinct begins to kick in. If puppies during this time have negative experiences with certain things (say inappropriate, rough, loud, etc children) then those specific impressions will impact said puppies future perceptions.
I truly believe that early, active (not passive) socialization is vital to allow the puppy to learn that exploring, interaction and recovery are all safe and enhancing activities. The key is to do so in a positive manner.
Many people think that socialization simply means to introduce a puppy to various environments, animals, people, children, noises, etc but if done so in a passive (unrewarding) or negative (traumatizing or *not* positive) manner, it increases the chances of the puppy developing fears, lack of confidence and in the worst cases, full blown phobias.
Owners should actively socialize their puppies while protecting them during this time and to do as much heavy socialization as possible throughout the life of the dog, more important in the younger ages.
I provide new puppy owners with a full sheet checklist of numerous, numerous situations, environments, items, things, floorings, experiences, etc that their puppy should experience. Things such as car rides, vet visits (only to interact with staff and receive treats), various flooring materials (each listed), various animals including appropriate puppies and adult dogs. Trains, motorcycles, skateboarders, joggers, etc, etc...the list is immense and also includes meeting people (positively, receiving treats and praise) in various clothing, beards, tall, short, races, clothing, etc.
Basically, the more and varied situations and experiences a puppy observes, interacts with and is rewarded for, the better equipped he will be in the future when he is faced with diversity. Our goal is for a confident puppy which connects new things with a positive outlook and the ability to recover and investigate when unsure.
I do believe that in the later months, as you mentioned nearing 6 months old, that another noted fear stage begins. But I would like for more puppy owners to not "schedule" socializing around these stated ages but to continue with active socialization throughout, but recognizing when their older puppy shows a bit more resistance to interactions and new experiences.
The idea is to not coddle or encourage resistance nor to "flood" (force a puppy to interact). A confident owner which has laid a foundation communicating to the puppy that they will keep their puppy safe and as stress-free as possible will create a puppy which will rely on his handler and feel confident that they will not be forced to feel uncomfortable or afraid.
My two current pit bulls cover both bases and interestingly, are a study in the importance of early socialization.
My male came to us at age 8 weeks and was actively socialized positively and thoroughly to more than most owners would ever dream of. He is a social butterfly, unfazed by sudden noises, constant loud noises, greets people of all sizes, races, ages, you name he's probably seen it and therefore he is well-rounded and extremely well socialized.
His litter sister came to us as a foster (adopted by us soon after) at age 5 months. She had never left her previous foster's backyard except for vet visits which resulted in shots and handling by strangers. She was never allowed to interact with adult dogs beside her mom. She had never seen anything beyond the wooden privacy fence.
The day I brought her home, she glued to my leg like velcro, guarded me from her brother and our other dog, a Miniature Schnauzer, did not respond to appropriate dog body language such as play invitations and growled at my husband when he met her for the first time.
I immediately began to get her out to socialize and realized how traumatizing it was for her. She is now 3 years old and extremely bonded to my husband and children and Cajun, and only about 3 regular people visitors. She is extremely fearful of being out of her safe zones such as our home, yard and surrounding areas and car and will growl at anyone who approaches her. I have worked extremely hard with counter-conditioning and positive methods to help her become more comfortable but she is one of the worst cases I have ever seen.
To top it off, because of her lack of early socialization, her coping abilities are lacking and is thunder phobic and terrified of fireworks and gunfire (which I cannot prevent as we live on acreage and people hunt here).
Had I just gotten her at an earlier age (such as 8 weeks) I absolutely believe that she would not have these issues or at least not to this extent.
My opinion is that owners cannot over socialize their puppies. They *can* do it inappropriately (bad experiences are impressionable) but they usually under socialize by a landslide.
Also, many vets still stand by the whole, "leave your puppy at home until he's gotten all sets of vaccinations" which really can hinder a puppies social development. There are safe places to socialize a young puppy and it is safe to do so if the owner knows what to avoid. Friends and relative's homes, appropriate, vaccinated adult dogs and other puppies, pet stores (carry the puppy/never on the floor), car rides (fast food drive thrus for a french fry!), neighborhood children play dates and any public places where the puppy will not be exposed to unknown doggie poop or remnant parvo should be explored.
The dangers of an undersocialized puppy far outweighs the potential of acquiring a deadly disease.
I say all of this without denying that there are many perfectly well-rounded dogs in the world that did not get alot of socialization, there certainly are. But why risk it?