I have saw and just answered a question that is a year old now by mistake. For some reason, even by looking at comments, answers, and edits, there is no reason why it should pop up at the top of the page as an "active" question again.
Should I have answered it even though no one is going to notice? Should I pay more attention to details to not answer these questions?
Should I have answered it even though no one is going to notice?
Pro Webmasters ranks extremely well on all search engines even the oldest of old questions can get regular views and improving the answers and questions will benefit the community greatly. It's also worth noting that many regular community members browse Pro Webmaster using 'active' and not by newest.
Meaning that even old questions that receive improvements should it be edits or newer answers then that question becomes active and sorted at the top, regularly old questions and improvements receive +1's. I personally 'REALLY' appreciate members improving older questions than newer because it makes our answer to question ratio higher, because let's admit it... no one likes clicking a like of a question only to find that no one has left a response.
That's perfectly fine. If you have an answer that is useful to a question, post it no matter how old the question is. The goal here is to accumulate knowledge for current and future visitors.
In some cases a new answer to an old question can add new information that was not available when the question was asked. We definitely want those kinds of late answers as they keep this site up to date and relevant.