Why do you cast downvotes on answers?
I've been on SO for only a few months. I've been doing a lot of browsing, playing catch-up with the huge body of knowledge that's accumulated here. Naturally I've been voting here and there along the way. I know there's already been this discussion on upvoting old solutions. But what's proper etiquette for downvoting old solutions that I come across (old being anywhere from a few months to years ago)?
Now that I think about it, there are a few separate scenarios:
I'm the first to discover something wrong with an answer. It seems like here I should downvote and post a comment, even if the post is years old?
The answer's been downvoted once or twice, but not seemingly unanimously. Maybe there's evidence of disagreement in the comments. Should I also downvote it, to show my years-late solidarity with its critics?
The answer was overwhelmingly downvoted. Given I agree with the downvotes, should I also get my kicks in, as long as I'm there?
For each of these situations, would downvoting be helpful to the site, or just unnecessary/amateurish?