If you reply to add new info to the thread, perhaps some new information that will help the thread along, then its fine, but if it is to ask a question or make a comment like "cool lock man, did you ever get it picked?" or "where did you get those saw blades" to the original people in the thread and it is older than 2 years, there's a good chance they won't see your new question or reply, so its just best to start a new thread if you have an actual question, and best not to comment and not resurrect a super old thread if you are just saying "cool lock bro".
Squelchtone