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