Similar to this message when reviving old topics:
Sometimes users reply to a months- or years-old post without realizing the post was that old. The topic itself may have recent replies, so there is no warning about reviving the topic in that case. However, I think it’s still worth warning the user that they are about to reply to a really old post.
Consider auto closing old topics?
EDIT: Oh. I missed that the topic was active and that they were responding to old posts in an otherwise active topic.
No. There is no way that you can make me read more carefully. 
Are you suggesting closing old topics that are still active?
The problem that I thought you were describing was topics that were to old to be useful. Also, Discourse does warn a user replying to an old topic.
No. Please reread what I wrote (and let me know what was unclear so I can edit it to clarify). I’m referring to people replying to old posts in an otherwise active topic.
To my knowledge, this is only done when the last post is old, not the topic itself. The picture I put is of that, which I mentioned since I’m proposing a warning message similar to that.
Yeah, I can see how this could happen on occasion but I’m not sure we’ve reached the rule of three yet. I’d be interested to see if other people have experienced this.
I wouldn’t say it happens often, but I’ve seen it a number of times in the couple years I’ve been using Discourse forums. I’m also curious to see if others have noticed the same thing or if my experience is a fluke.