No, that's rude. You can at least post a comment to the answer and give people a chance to improve their answer if something has changed. Or, better yet, just post the updated information in the comment. Or... even better, edit the information straight into the answer! That's why this is a community editable place. Information does change.
If you're really greedy for reputation: sure, you can post your own "updated" answer, but surely don't downvote other people just because their answer is out-of-date. No one goes through all their answers to find the ones that need updated with new information.
Anyone can look at the date the answer was last updated and logically figure out "hey, this was posted a year ago, maybe something else has happened since then."
This answer of mine stating that a specific CSS function cannot be done at this time will eventually become outdated, whenever CSS4 is officially released and becomes supported in browsers. However, it will still always be "correct" in the context of the question, because at the time we were stuck with CSS3 which didn't support that functionality. Since this question specifically refers to CSS3 in the text, submitting a new answer wouldn't be appropriate, but it's always nice to add a simple comment noting that the functionality is now supported, whenever that comes to be.
Or, see the comments below for other examples.