I tend to find new users are the ones that necro old threads in their first post.
There is no rule, per se. However, a couple of things to note:
1) A lot of spammers necro threads, because the are picking threads mostly at random. Don't make yourself look like a spammer.
2) Don't expect, or act like you are owed, replies from old posts.
3) The "forum list" view in the Community menu is chronological. If you start there, you are unlikely to necro a thread.
4) If you find a thread from a search on a topic, just get in the habit of checking the date of the thing. Some folks respond poorly to necros simply because it is an indication that you aren't paying attention - and they find the combination of you can't be bothered to check a date, but figure folks want to hear what you have to say, to be kinda rude.
Ask Jeeves!More often than not (IMO), it's because they found the thread through google, bing, duckduckgo, um ... altavista?
And that's why they are posting there.
Generally I would recommend acknowledging your own necromancy when you knowingly do it, to preempt the snarky comments of people like me (often a good opportunity to explain why you thought the thread deserves revival as well). I'm mostly pro-necromancy, I just find it funny when someone interlopes into a long dead conversation and doesn't realize it, particularly if they come in hot arguing with someone who hasn't been seen in years.
The "Similar Threads" list below whatever thread you're looking at sometimes recommends long dead threads, and I think most acts of accidental necromancy by regular visitors stem from this (most of the rest presumably have to do with someone following a google search).
We have a Lich in our midst!
Maybe a few of these things:
This site doesn't make it particularly easy, and regulars have had to get used to that. The biggest problem on this front is that even when you click on "Community" to get recent posts it doesn't tell you if those posters were thread necromancers (you have to actually look at the rest of the thread), so when someone, often accidentally, revivifies a thread, it becomes much easier for anyone specifically watching threads with recent activity to hop onto it as well.IMHO, yes it can defeniitelly be inattentiveness, but still, checking the date should not involve one to actually have to go out of his way to check it. The info should just be 'right there' already, without needing "scroll to top/bottom of page (some pages are REALLY long) and then click to First or Last page (many threads are multipages especially popular ones) and then click back and scroll again to return to where you where. The info should just be "right there" with at most needing a single extra user actively-made step.
This site doesn't make it particularly easy, and regulars have had to get used to that. The biggest problem on this front is that even when you click on "Community" to get recent posts it doesn't tell you if those posters were thread necromancers (you have to actually look at the rest of the thread), so when someone, often accidentally, revivifies a thread, it becomes much easier for anyone specifically watching threads with recent activity to hop onto it as well.
But that seems to just be vanilla XenForo, and if the folk running the website don't care about thread resurrection it shouldn't be on them to create and implement a bunch of custom code to prevemt pr discourage thread resurrection. Yes, this is an easy website to accidentally necro on, but there are also usually no real consequences other than the embarrassment you feel when someone makes a "lo, the dread necromancer hath arrived"-type snarky comment a few posts later.
Personally, as someone whose first attempt to post on another rpg website before coming here was met with a stern "formal warning" from a mod for daring to post on a six month old thread that was at the top of my google search, I appreciate ENWorld's laissez-faire attitude on thread necromancy. It's certainly more welcoming to those not in the know.