hedwards: I don't want to waste my time going into a thread that's dead because some wiseacre thought it was cool to post some trivial tripe at the end and drag people into {a} post.
JMich: This is your main problem, not the age of the thread.
When asked about necroposting, I do refer to the threads as conversations in a bar. You hang with your friends, talk about relationships, movies, games and other stuff. If someone wishes to revive a topic you talked about in the start of the evening with "Yeah, that car is really nice", you would laugh at him, but if his comment was "You know, the problem with your car's engine may be in the gas filter" you would gladly return to the topic.
So it's not the threads, it's the posters. Locking the threads wouldn't help, we would just have more threads about the same thing, and you wouldn't be able to find the information you recall seeing in one of them.
A "The last post in this thread was over a month ago, are you sure you wish to revive the discussion?" could work, assuming people would read it.
I don't really think that locking is the only possibility here, it's just the most commonly used method. It's something that could be implemented in the forum software efficiently. I fairly regularly come across topics where they've been locked due to age.
The other issue with necroposts specifically is that they tend to go off on tangents that have little to do with the original post. At which point you might as well have just started a new post rather than dig up an ancient post and reducing the benefit that might come from referencing it in the future.
Having the forum software give necroposts a different color, change the title a bit or provide a reminder that it's a necropost would all be acceptable solutions. I just find it to be irritating to respond to something only to realize that the post was from years back and the poster I'm responding to probably no longer cares about the topic, assuming their even around any longer.
Another issue is that locking topics also cuts down a bit on the spambots doing SEO using posts on old topics. Yeah, that isn't much of an issue here, but it's another reason to consider locking excessively old topics.
IronArcturus: I would hope that older threads would not have to be "locked down" at all. It doesn't make sense to do that since some current threads were created years ago. But an indicator might help to show people that the thread is older.
AFAIK most of those threads have been in continuous use for years. In other words even if we were locking threads based upon creation date, they would just be created anew anyways.
The threads we're talking about in here are ones that were probably never terribly popular and haven't seen any new posts in years. Then somebody posts a trivial and largely pointless post at the end and it gets this sort of weird undead response before people realize that it was a necro thread.