Adding an answer, or editing a question or an answer, bumps the post no matter its age. Comments do not. This is by design and makes sense to me -- if there's a new answer or some other improvement, we want people to see it.
Because edits bump, it's important to make them meaningful. Don't just fix one typo; look over the whole post and see if there's something else that ought to be fixed while you're in there anyway. Edits that are too minor usually won't be approved (if they go to a review queue), and if you make a habit of such edits you might be asked to change your behavior.
That said, we're trying to make this the best place on the Internet to get answers to questions about world-building, so please do improve old posts -- responsibly and bearing in mind the bump.