I asked this question on meta.SE, it got marked as dupe for this question, I'm posting it here because it has more value here.
This question is 4 years old, someone commented:
Looks like the old accept is no longer valid... this is the reason accepts should float after a certain point in time... – Aaron Hall
I don't think that floating accepted answers, is the solution, because 4 years ago the answer was perfect.
My suggestion is that all questions are older than 2 years should have a banner like the banner of the duplicate question that says:
This question is old, the answers given may no longer work, you could sort them by date and try the newest of them and if none worked please flag the question to be archived.
If a question is flagged as old, it would still exist because it is still useful, but newer questions cannot be flagged as duplicates of the old question. And old question cannot resurface.
My proposal might be a good proposal for all stack exchange sites, but I think it's more useful for sites like Ask Ubuntu, my question is tagged 13.10, I think it's fair to keep it that way and just archive it.
If I unaccepted the accepted answer, I have no way of verifying which is the best answer and future users will look for votes, it means they would still try the old answer.
An automated solution based on the date and tags used is the way to go, on sites which suffer from having too many old questions.
Regarding the question: Automatic visual indication of old questions:
There are two answers. I disagree with the accepted answer saying users don't read.
I agree with the second answer saying that there's no way of determining if an old answer still works.
But I believe that such a visual indication is needed on Ask Ubuntu and others, because there's a high chance that the answer no longer works, in fact Ask Ubuntu refuses to answer EOL questions for that reason