Who sent you that PM? It seems rude for what's a minor misdemeanor, made in good faith. Sorry if that was one of our mods.
And yes, the forum guidelines do advise against posting in old threads, as most of the people involved in the discussion will probably have left, or at least forgotten about the thread.
Personally, I don't see the problem, if you've something new to add to a discussion, then bringing the thread to the top of the list will prompt members to read it, and perhaps discover something interesting in the process. In terms of cluttering up the forum, it's no worse than starting up a new thread. But whatever, it's not really a forum rule, more of a guideline, plus there's lots subjective about it, such as when a thread becomes old, etc.
Incidentally, I wouldn't be so sure about your "oldest member" claim
http://www.abandonia.com/forum/index.php?showuser=3247
(Yes, that is his age, he still fairly active on the forum too)