Summarize:

A kitten of any age can benefit from exposure to new people; however, kittens between two and seven weeks old have been shown to benefit immensely from socialization. If you have purchased the kitten from a breeder or pet store, you may not have the kitten until he is six or seven weeks old, so it is important to bring him around people right away. When you bring home a new kitten, give him a little time to become comfortable. (This length will vary from kitten to kitten, but a couple of days is a good rule of thumb). Once he seems comfortable around you and in your home (he is exploring on his own, snuggles with you, and is eating), begin to invite guests over. Kittens will grow accustomed to people if they are exposed to many different types of people when they are young. Invite over friends of different sizes, shapes, and ages. Invite people of various races and genders. Additionally, it is good for him to encounter people who look "unique" for one reason or another (such as people who use wheelchairs, people with beards, or people who wear hats and/or glasses). Make sure that the people around your cat are kind and gentle. Avoid very loud noises. Be careful with young children, who may not understand how to handle a kitten. Anytime your kitten actively approaches a new person and interacts with them, reward the kitten with affection and praise. It is important to teach your kitten that this is a desired behavior. You may even want to offer him a treat.
Know your kitten's age. Provide a safe environment. Expose your kitten to diverse people. Reward him with praise.