The best place to start was 5 weeks ago. There is an important developmental stage that occurs in all dogs between the ages of seven and sixteen weeks of age. This is the time when puppies learn (whether we teach them or not!) which things in life are good and which are not. Now commonly referred to as a critical socialization or developmental stage, this is a rare window of opportunity for us to teach puppies to become confident, psychologically healthy dogs. We do this by exposing them to the myriad of things they will encounter in their lives: people (young, old, different colors, male, female, with mustaches and beards, with eyeglasses, with hats, with mailbags), other animals (cats, dogs, horses), sights and sounds (trash trucks, blimps, fireworks, thunder, lawnmowers, weedwackers), etc.
Any socialization done after that window shuts at 16 weeks will be far less effective than socialization done prior to this. This is one reason why Puppy Socialization classes are so popular...you have a safe environment in which you can socialize your puppies starting at a very early age. You still have a lot of work to do outside of puppy classes, and must work very hard at finding safe dogs and safe locations in which your puppy can learn.
I realize that there is a potential risk for parvo. First, parvo is not necessarily a death sentence in an otherwise healthy puppy. Second, faaaaar more dogs are euthanized from lack of socialization than ever die of parvo. You are much better off being proactive when your puppy is under 4 months of age and risk the parvo, than having to spend the next 12 years with a fear biter.
Having said that, selecting your breeder is essential!!!!!!!!! Ease of socialization is largely genetic and if the sire and especially the dam (who will be raising the litter), are calm and socially savvy, then your puppy is much more likely to be so also. Socialization is always important but the ceiling on how much your puppy will tolerate is genetic. Many labs are kenneled and have few social experiences and grow up to be wonderfully adjusted dogs. On the other hand, many 'one person' breeds such as Mastiffs can be taken out extensively and still become wary of novel people or dogs. Sadly, I am seeing low ceilings on many labs and this is not a good trend!!
I also see lack of impulse control being called lack of socialization quite frequently. Teach your dog early on that you have better games and more terrific rewards than any old dog or visitor to the house and he will display better manners around them. So many dogs lose their head when they see other dogs simply because they have not been taught that it is possible to contain your excitement in a more appropriate manner, and because they have learned that the best way to get an environmental reward is to simply bowl over their owner to get to it