and what are the criteria for doing this?

Forum topics are 'archived' after a certain period (a year, I think) and are automatically closed at that time.

Other reasons why a topic might be closed:

1) Duplicate to another active thread
2) Contentious topic that doesn't add value to the community
3) Non-travel related thread on travel specific forum

The mods rarely - if ever - close a thread without some kind of explanation. If you look at the last post on a closed topic you should see a description of why it was closed to further comment.

Usually, old topics are closed down so that they are not resurrected long after the original poster has asked the question.
Threads are also closed if there are duplicates. Sometimes if the topic becomes derailed or discordant and not beneficial to the community, it is closed then as well.

Apart from that our forum is set to automatically close topics that are a year old, and it runs a check on it once a month.

Hope that helps.