Hogwarts as far as we know it the only wizarding school in England, and every indication points to Lockhart being British, how is that none of the adults would have recognized him, as a student, I'm sure he must have been a complete flop as a teenager as well, wouldn't they suspect he's a fraud? He just didn't stand out and he wasn't hard to forget and maybe he didn't attend Hogwarts at the same time that the other adults within that book. If Petunia or Dudley turned out to be a "late blooming witch/ wizard, do you think Vernon would have divorced his wife, or disowned his son? What would have been his reaction to both cases? I don't think so, he would have just got used to it. He hates the magic world because it's related to Harry and mostly because Petunia hates it. There are obviously vampires in, Harry Potter even if Rowling didn't really touch up on the subject. Do you think that if a wizard becomes a vampire they lose their magic? They would just be animagus. Do YOU think that the body of someone with a Horcrux ages? Yes, but at a much slower pace.