For forum games, posting on old threads is totally ok ^^
In general, the "don't bump old threads" rule usually only applies to threads that aren't relevant anymore. For example, a group advertisement thread from 2011; it is very unlikely the group is still around, so there's no point in it being bumped up since anyone interested in the group will likely never get a response (there is a small chance that the person that ran the group is still around, but if their group disbanded years ago they wouldn't go accepting or declining the poster into the group)
However, perhaps it's an old game discussion thread about random/funny occurrences in the game. This would still be considered relevant because many crazy things happen in-game c:
I hope this is clear, but if you're ever uncertain about a thread feel free to pm a staffer about it c: