Should there be a warning before a user attempts to modify an older post?

Question

A second ago I was one click away from commenting on a question from 2012, because I did not notice it was so old. Incidentally, I stumbled upon it via the side bar.

As far, as I understand, "late answers" (and, by extension, comments) are frowned on. I would not suggest disabling commenting on them. I suggest some kind of a visual clue.

1. A deeper shade of background. Of a JS-popup "This question is from 2012, do you really want to answer it?" might help.
2. Would it make sense to filter the questions that pop into sidebars by age?

Answer 1

As far, as I understand, "late answers" (and, by extension, comments) are frowned on.

No, that’s not correct. Stack Exchange is not a forum and we do not frown about thread necromancy like most forums (which does not even make sense for most of them). If the information in the existing answers is outdated, incomplete, or just wrong, you are encouraged to write a new answer. Not doing so would be like refraining from editing Wikipedia articles that haven’t been touched in a while. There are even two badges for providing good late answering: Necromancer and Revival.

The reason why we have a review queue for late answers is to get another check on late answer (of new users) to compensate for the decreased visibility due to the question’s age.

Answer 2

I don't think that late answers are frowned upon. Where did you get this impression? Yes, it frequently happens that late answers contain spam or questions by new users instead of real answers, and these are certainly frowned upon, but a good answer to an old question won't cause any issue (it's a pity, though, that a late answer may not get the attention it deserves).

For instance, I actually answered a couple of old questions on Cross Validated – without realizing they were old – and one of my answers was then accepted, even if it came some three years later than the question.

So, no, I don't think there should be any mechanism to discourage users from answering old questions.