The general rule of thumb is to not necro years-old threads. But it can depend on a few things: 1.) Is the topic still fresh or relevant (you might see this sometimes in the Activism, Current Events, and Hope forum or AW Roundtable where new information has come out); 2.) How old is the thread? If the last post in the thread is several months old, would it not be better to start a new one? Generally, it is much better to open a new one. 3.) If the original post was a request for information or help, is it likely they have received the information or the question is for some other reason no longer relevant? In that case, don't necro it.
Different mods have different takes on this depending on the natures of their rooms, so when in doubt it's probably best to drop the room mod(s) for that room a quick PM and ask. For instance, I mass-lock old, old threads, particularly in the Contemporary and Roundtable rooms—so if a thread is approaching a year old or so, it would be better to ask.