A spam post, low value post, off-topic post should be deleted whenever it is reported or seen by moderator. The age of that post is not important to decide deleting it or keeping it in one topic.
I disagree that limiting the age of post to 6 months + to consider deleting it or keeping it. Considering about its quality and off-topicness, contribution for discussion in that topic, and making a decision to send it to trash bin.
Forums are living with their own life. What happened a couple of weeks ago is usually old already and what happened several months ago is ancient. If a topic is not some kind of educational or containing rules, likely mostly no one will read anything that old as a half of a year. So necroreporting is only making moderators work too hard but has little sense. It's much more important to keep clean most actual topics.