Whether you bring home a new puppy or you bring home the older, more mature version, don't overlook the importance of socializing a dog. Well socialized dogs make better all round pets as they have more acceptance and less fear of the world around them.
A socialized dog will still be appropriately protective. Many owners of large breeds of dogs want their dog to not only be a good pet but also a source of protection for the home and family. As their pack family, dogs will naturally want to protect their home and family. Even if (hypothetically speaking) your dog wasn't crazy about you, he would still protect the home because that is his source of safety and food. Thus, socializing the dog is not going to prevent him from warning of danger or disallowing the entrance of a stranger while you're not home. He will do his job no matter what.
An unsocialized dog will be more apt to threaten to eat the dogs in the neighborhood (or at least sound like he will) as well as prevent friends and relatives from entering the home. An unsocial pup is less likely to accept a new babyor allow children to visit. An unsocial dog is not going to present with behavior that will be acceptable to leave the house and go out into the world.
DO NOT MISTAKE SOCIALZING A DOG WITH RANDOMLY PLUNKING HIM AT THE PUPPY PARK WHILE YOU GO OFF TO CHAT WITH FRIENDS.
Unsocialized dogs require tranquilizers just to go to the vet's office. There are numerous hang ups unsocialized dogs have, and each of them makes them more difficult to own, more hazardous to those around them, and less loveable in the long run. Unless you are planning to never take your dog anywhere, skip all his routine vet visits and never get him proper medical care (in which case then quite frankly you should own a goldfish instead of a dog) then you are going to need to socialize him regardless of his age, size, or status.
The only cautionary factor to this socialization concept is Parvo Virus. Parvo, which can live in the ground, is quite costly to treat, and only runs about a 50% chance of being effectively treated.
Puppies should not be actively socialized until after they have received their Parvo virus vaccination, which is usually around 12 weeks. Before then, you can invite people over to the house with their dogs. If their dog poses no threat, and introduce him to the visitors which come in and out of the home. This is an excellent idea even after outside socialization has begun, as it teaches him the appropriate manners when guests enter as well as keeps him from becoming aggressively concerned with the people who enter his home.
Socialization can take place in numerous forms. You can take your dog out and about and let it happen naturally. You can take weekly trips to the park or you can take him to puppy school. Any of these methods can give a dog a healthy dose of social experiences for him to develop his personality well rounded. The main goal is to get him around new humans, new dogs, and if possible a few cats to learn how to be a good friend to those around him.
Don't mistake socializing a dog with randomly plunking him at the puppy park while you go off to chat with friends. While new friends will be an additional bonue for you, the puppy park is a chance for him to learn proper puppy etiquette, which includes refraining from pawing people and refraining from urinating on other dogs (a sign of dominance). Being a good pet owner means staying involved with him even while he is romping and hopefully playing nice and making new friends.
Last but not least, part of the importance of socializing a dog is to teach him how to relate with other dogs in their own language. Unless dogs are getting rough with each other, don't hen peck him to death. Most dogs will tolerate a lot from a puppy, but it is okay for them to tell him when he has pushed their limits beyond reason. Naturally, you don't want your dog to get hurt, but you dont' want him to turn into a 'prince' either. Not to mention that other dog owners will usually be quick to intervene if their dog is the one that is out of line.
The importance of socialzing a dog is high enough on the list that, young or old, it will help to shape his personality. While he will still want to keep his home and family safe, he will be less likely to be a nuisance as events in your life change. He wants to be a good guy, but he needs your help in understanding the ground rules.