Advice on avoiding necros

How old a thread/stori is it acceptable to comment on on this site? Or does it depend on context?

For threads these days, if it’s off the first page it should be left alone, and please think carefully if it’s older than six months or so.

On stories, there’s no particular limit. People often like being reminded of stories. That said, I consider it preferable to say more than just a few words if the story is getting older.

Define “first page” as not everyone’s first page is the same.