**No**, that's very rude in my opinion. I think there are better alternatives.

### The answer needs additional details
If the original answer is correct as it is at the time of posting, but now requires additional info to cope with newer systems/updates, you can do one of the followings:

 1. Edit the answer yourself and add the details.
 2. Leave a comment with the details and suggest the answerer to append the details to the answer.

### The answer is completely obsolete (but was correct at the time of posting)
Just leave a comment saying it's out-of-date or just leave it as is. I believe users will see the date of posting/last edit and figure out that the answer might not work with a newer system.

If you have a up-to-date answer feel free to post it. If you think you have a (Ask Ubuntu) link to an up-to-date answer,  edit the outdated answer and append the link.

>Another way of doing is to tag the answer with the Ubuntu version when possible. That is, add a sentence to the top of the answer: `## Ubuntu xx.xx and older`. But this would require to update other answers as well and specify which one is for versions newer than xx.xx. This helps users skip the outdated answers altogether, and look for the appropriate one. For example: https://askubuntu.com/q/256013/8698  
>
>([*suggested by Dan*](https://meta.askubuntu.com/questions/17523/should-an-answer-be-downvoted-if-it-is-outdated-and-at-the-top-of-the-page?cb=1#comment36494_17524))

### The question itself is obsolete
You can vote/flag the question for closure. There is an option under *Off-Topic* which goes like this: 
>This describes a problem that can't be reproduced that seemingly went away on its own or was only relevant to a very specific period of time. It's off-topic as it's unlikely to help future readers.

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Of course if the answer is completely wrong/rubbish/potentially dangerous to begin with, downvoting (maybe also leave a comment pointing out the problem(s) with it) is the right action to take.