I was going through the review queue on another site and ran into a question that needed formatting. Then I realized it was in the answer queue, and only tangentially related to the question it was posted on. I looked at it and it hadn't been active for 4 months. Is it better to leave such posts dead or flag them for deletion? And should I edit it so it at least looks acceptable until it's removed? (For the record, I did flag it, but didn't edit it).
One of the things that makes the Stack Exchange sites better than anything online, that came before, is the marking down or elimination of "noise" and bad information.
So, yes, even if the post is old, or heavily upvoted, or even marked as the answer; it should be flagged if it is not an actual answer.
Things slipped through that shouldn't have, and SE standards have crystallized a bit in the last few years. Just because the burglar has been living in your house for 2 years doesn't mean you shouldn't throw him out! ;)