As a trainer, I talk a lot about the critical socialization period in puppies under the age of 4 month. So if you get your puppy over the age of 4 month, or if your puppy did not meet all their socialization needs before the age of 4 months, does that mean that you lost your opportunity to have a well behaved dog? The answer is no.
Dogs continue learning throughout their lives, the only difference is the speed in which they're receiving and processing lessons. With the right techniques, you can shape a dog's behavior as well and change the dog's demeanor well into adulthood. Your dog's demeanor doesn't "solidify" until around the age of 2 - 3 year old.
This means that you can adopt a dog in the juvenile period and sill be able to improve it's confidence and behavior. It also means that if you only put in training and socialization while your puppy is under 4 months of age, it's very likely that your dogs behavior will change as your dog approaches adolescence. In order to ensure that you maintain wanted behavior as your dog ages you must continue socializing and training.
Dogs in the juvenile period should have the opportunity to play with other dogs, meet new people and experience new places. As is in the critical socialization stage, socialization is not simple exposure. Dogs are learning every minute that they are awake. If they have an uncomfortable experience, they may be learning that they probably shouldn't do what ever that was again.
I personally hate dog parks. Yes, I want my juvenile dog to play with other dogs, but I want that play to be appropriate and with dogs that I know are tolerant and have good communication. There is no way knowing what you're going to get in a public dog park. I have seen too many dogs that shouldn't be interacting with other dogs at public dog parks.
Instead of going to public dog parks consider coming to a Juvenile Jubilee at our facility. At this social we will be concentrating on appropriate dog play, understanding behavior and stress and maintaining the value of each owner.
Other alternatives to dog parks are off leash hikes. You can meet other dogs on a hike and allow for some short interaction before moving again. You may also become a member of the Humane Society of Silicon Valley dog park. This park is better because it's for members only and all members must attend a behavior seminar organized by Cecilia Sternzon, CPDT-KA. Even tho the HSSV park is still a fenced dog park, it has much more educated patrons than the local public park.
Consider that your dog is learning from every interaction they have with dogs, people and places. Be sure that they are learning from experiences that will shape their demeanor and behavior for the better.