The critical sensitivity period is between 8-12 weeks. Its called 'fear phase' but don't take that literally! Its a time of development of the brain where adverse situations (e.g. bad meeting with unfriendly dog, being smacked etc) can 'imprint' on the puppy and be carried as fear through life. All experiences during this time should be positive ones.
Gwen Bailey explains this beautifully in 'The Perfect Puppy'
Some breeders do not let their pups go earlier (especially older breeders) as they felt mum should play a role for all this time (not true, but more in depth than I can explain here), some because of vaccinations etc
The BEST time of socialisation comes to an end at about 12 weeks, so you need to ask your breeder how they socialise the puppies. If they have them used to a car, bikes, joggers, older people, people with glasses, children etc
Your job will be easier if its done (and it should be done) otherwise you are best waiting to introduce these things AFTER 12 weeks when your baby is confident in his new mummy, YOU.
This is a huge subject, one I feel very passionate about and I am happy to explain some more if you want to pm.
PS the other sensitive period is between 16-21 weeks. A pup doesn't have to display anything. Its just an awareness of the stages of their brain developing
