A lot has to do with their age. Mine is friendly to everyone but he is only 12wks and thats how i expect him to act at that age. If you had a older dog acting like that i would possibly be concerned. Also you can control WHEN he meets someone by putting him on a leash and allowing him to meet someone either by working for it (sit,stay) whatever your heart desires.
Depends on the dog. 24 months is the average when they mature but some could go as long as 3 years before they mature as with a lot of Czech/DDR lines but you can find those in any lines thats why it depends on the dog and HIS lines.
Your pup is 12 weeks old? At this point, you should be encouraging your pup to meet and interact with everyone (within reason) that you possibly can. You want a well socialized GSD. Loyalty will come. As long as you treat your dog well, train him, and love him, he will be as loyal as anything. And he is developing a strong bond to you as I type this.
Every time my puppy shows an healthy interest in someone (and the other person looks willing to oblige), I walk with my dog to the person, encouraging this behavior. We want our dogs to like other people, to be confident around others. This is a good healthy behavior. My adult GSD loves other people -- just adores them. And he is simply the most loyal dog I could hope to find.
If anything, the more confident you help your dog to be, I think it's likely the more loyal he will be. Not attached to you because of fear or lack of confidence, but proud and loyal because you're a team.