Everyone has a slightly different rule of thumb for it, but here is mine:
If your post on an old thread would be to give the OP advice, don't bother. Once a month has passed the situation is likely to have changed and giving advice is too old and cold to be of any use to anyone.
If the thread had arguments or flaming in it, we don't want to see it again. Best not to read it or form an opinion that makes you want to post. If it's old and cold enough, you raise the points of interest in a new thread, as LeagueGirl indicated. I realize that this technically violates the rules, but the rule is meant to keep people from spamming, or spreading a flame war. It is not meant to keep people from revisiting a topic from a new perspective more than a year later. If it hasn't been very long, accept that the discussion may be raw and should be left alone.
If the post is simply old, as in over a year old, people may get confused when it comes back so, again, best to leave it alone.
The last I think is a key issue for people: they get confused by the old threads, and often reply as if it's new, only to discover they had replied a year ago. After doing that they feel they've wasted their time and now they are annoyed.
But if a thread involves lots of detailed and interesting information that was shared in an intelligent manner and that would be a royal pain to reinvent ... it might be OK bring it back. You will still have the issue of confusing people, but at least you'll have a good reason for it. In fact, I would state that reason in your post that brings it back. It's really weird to read a conversation as if it is in real time when there are months or years between posts. So bridging that helps ease things. Or, perhaps better yet, start a new thread and link it back to the old one.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).