tl;dr: Yes. No post is too old to get an update.
- StackExchange is Q&A, not a forum. It's for getting questions answered, not for talking about questions.
- The really basic goal is awesome answers to awesome questions. Answers that are useful to the person who asked, but also answers that are useful to somebody 5 years later who doesn't have to ask because we already have the definitively awesomest answer.
- Making things better is good.
- Ridicule isn't allowed. Most of the behavior you describe would get deleted by a moderator (such as myself) pretty quickly, or at least downvoted.
- If you have a different question that's only sorta related to another question and the answers won't answer your question, you'll probably want to post a new question, but include a link to the other question. (and maybe also explain why answers to that question won't help you)
If you have an answer that's better than any of the existing answers on a question, please do give us your better answer. If there's new information that invalidates an answer or makes for a better answer, please update.
If there's an existing answer that's close, you may instead want to instead comment on that answer, or if you see an "edit" link on the answer, edit the post to improve it. Do try to only put things that answer a question in the "answer" area, and anything else should be a comment.
In fact, what you're asking about is such a good idea that there's two special badges you can earn by doing it: revival and necromancer.
One small caveat: newer questions tend to get more attention, so new answers to old questions sometimes don't get as many upvotes as they deserve. Personally I think this is a small problem in the StackExchange type system that may get better over time. I specifically check for new activity whether it's on new questions or old questions, but not everybody does that.