Is it any use answering or commenting on a question that was asked years ago? Or is it just 'un coup d'épée dans l'eau' (a strike of the sword at water), meaning a complete waste of time?
If you have an answer that better answers the question than any other answer already there, then please post the answer. Questions get more views the older they get, and new information comes to light all the time, so it's in the best interest of the site to post a new answer when the answer has more up to date information.
New answers bump the question to the top of the front page, so your answer will garner new attention to the question, as well.
There is little to no point commenting on questions that old. Unless there is something really insightful you can ask of the person who posted the question, that will make the question useful, it's unlikely to be worth yours or anybody else's time to do so.
Commenting on older answers might be helpful if you have some constructive criticism and the person who answered is still active on Stack Exchange.
Relatively new participants at this site may not be aware of how much traffic older questions get; but anyone who has asked a question of broader interest than "Is this sentence correct?" can see (by referring to the Questions data recorded on their 'Activity' page) how the number of views the question has drawn continues to grow.
The substantial number of Necromancer badges awarded on English Language & Usage provides strong evidence of the value to later site visitors of posting well-conceived new answers to older questions. This silver badge, awarded when you "Answer a question more than 60 days later with score of 5 or more," has been earned 975 times on EL&U, even though such answers have to compete against longer-established answers and (in many cases) accepted answers.
English Language & Usage is an unusual Stack Exchange site in that it hosts many questions that ask for factual answers but that nonetheless may be answered in multiple useful ways. Even when a question has one or more good answers, it is often entirely possible to introduce a new, nonrepetitive, good answer that makes the collective EL&U answer (which consists of all of the answers on that page) even stronger.
The test I use in deciding whether to post a new answer to an old question is this: Does the answer I have in mind provide new and useful information or a different and interesting perspective for understanding the problem that the original poster's question asks about? If so, it's worth posting.
The original poster or the age of the question is not the point at all. The point is the quality of the question and the quality of the answers.
When you post an answer to any question, here is who benefits:
- all users of WorldWideWeb, who often Google a question and find the answer on a StackExchange site
- all users of StackExchange and the particular StackExchange site
- the individual who asked the question
- the individual who writes each answer because sharing knowledge enhances the knowledge of the person who shares it.
So the person who asks the question is not the only beneficiary of the answer. If they had that question, it is likely many others have that question. That is why StackExchange is on the Web rather than the questions just going to the original poster through email.
Further, how old a question is doesn’t actually tell you whether answering it will benefit the original poster. Someone can ask a question on a StackExchange site, then 5 minutes later find the answer somewhere else, and then you come along and answer a 10 minute old question that is already irrelevant to the original poster. But your answer may help another StackExchange reader a half hour after that, and somebody who finds the answer through Google the next day, and then 200 people who find the answer through Google over the next month. And then 2000 people over the next two years.
StackExchange is a reference book. The questions are just a way to create a new chapter and add to the book. The book is written for everyone, including people who will read this page 5 years from now.
YES. Yes. Yes. Yes.
StackExchange is a site designed to share expertise. Sometimes I may ask a question and get well-intentioned answers but find that none of them were really helpful. I'm sure OPs would appreciate new answers if that answer satisfactorily answers their question or solves their problem. (There is nothing like completeness!)
Alternatively, even if the question is too old for the new answer to benefit the OP, desperate answer-seekers - particularly programmers - may just find that golden answer you just posted to an age-old question. As a programmer, these golden answers are essential - I may be searching and searching, and finally... I have an answer! It works! It's a fantastic feeling.
So, in conclusion, yes! Google is a significant part of SE and your contributions might just help that person who has been searching for days for an answer to even a simple question. After all, a question left unanswered is a question that bugs and lurks around the mind.
The date is merely a timestamp.
(Oh, and one last thing: If an answer has become outdated or no longer works, then it should certainly be rectified!)