Necroposting kind of goes both ways. If done properly, it's not inherently bad. Maybe the OP wants to follow up, or the thread provides a solid base of discussion to build upon. Although, as user accounts get deleted, older threads might have a broken flow so other posts in the thread are now out of context. Depending on how old the thread it, the information might even be out of date. So, it might still be a good idea to start a new thread anyway.
When it becomes a problem are personals ads which don't age well.. I'd even go so far as to say if the thread is more than 3 months ago, don't bother replying. this is because there's no practical way to track whether the thread was successful, if the OP is still interested, still single, etc. The OP can always make another thread.
I get the sometimes intense desire to find someone, but bumping old threads, especially when the OP is long gone, not only doesn't help but makes the personing doing the necroposting look bad.
Another time when it's a problem is if someone bumps an old thread just to start a problem when none exist. I almost locked a recent thread where someone did just that, but I decided I would wait to see how it played out.
16 hours
