Aw, zombie threads. Yes, they can be annoying. I for one have been reading a thread, found one post and thought "I totally agree with this" and then realize I actually wrote it years ago (I guess its better than thinking I was an idiot. )
I've had this happen to me!
Maybe someone could PM the newbie and let her know this is generally frowned upon?
I wouldn't say, "let her know this is generally frowned upon"--that sort of unsolicited advice, especially if worded that way, is not that well received. I know; I just received a "you're doing it wrong" PM from someone at a forum I'm on, and I did not appreciate it at all.
You might have better luck if the tone is, "Hey, I noticed that you've been commenting on a lot of the threads, and I wondered if maybe you didn't realize how old they are," but I think you still run a risk of alienating her. That's unsolicited advice.
And I do like the comment that Lynn2000 suggested (and I think a public welcome like that is better than a PM):
Sometimes another member will add the next post in that thread, saying something like, "Hi, welcome to the forum! Just so you know, this thread was started in 2007. I think BeadLover10 resolved her issue, but if you'd like to discuss the topic again/ask your own question, you could start a new thread."
A similar wording could be used in PM--but please don't chastise or "advise" people via PM.
Otherwise, I think you just wait for it to die down. New people sometimes have trouble fitting into the group--they'll probably get it eventually.
One of the things that happens on a forum is that people may consider it to be a resource. So a thread on non-payment may pop up on a search for someone, and her advice might actually be relevant. (In some situations like that, I might have started my comment with, "I know this is old, but I had a thought that might be useful to someone who comes to this thread later." Or I've added a comment like that to the thread right behind the newbie, much as Lynn suggested.