It depends. The best policy is that if you chose to do so and the source is contemporary; ask. If it's something that you see online, at least credit it when you post it.
There's a fair amount of mockery surrounding responding to old posts, which is unfortunate but common. If you do it on accident, there's no reason not to have a laugh about it. If you truly intend to respond to an ancient post, make your response useful by paraphasing why you chose to do so. Like "I know this is a zombie thread, but the information in it was recently useful to me and I wanted to share with the rest of you".
Mentor, well collectively this whole group is everything you need. It sure works for me!
Welcome aboard. I have designed buildings in Hailey, Ketchum and Stanley. Beautiful area.