> It might be helpful to have the age of comments listed

That's... currently the case?

> red highlight for comments > x years

I don't find age has a 1:1 correlation with it being outdated. If some advice doesn't make sense to me, I'd look at the dates of this and other answers, because most often there will be newer answers (lower voted because they haven't existed as long / aren't seen as much) and/or comments added to the answer indicating how to do it in python3 or whatever the new thing is.

Sometimes posts from 2009 help me, sometimes posts from 2018 are outdated. Maybe this could work if a time limit is configured per tag, but even then, I expect it wouldn't be very helpful.