From Google Groups Help:
You can reply to any message in Google Groups that was posted in the last 60 days. After 60 days, the option to post a reply to the message is removed. In Usenet proper, inactive articles are usually retired (removed completely). Removing the option to post a reply to a message more than 60 days old simulates this feature in Google Groups.
What I want to know is whether it is possible to override this default behaviour for your own Group and allow replies to be posted to older posts - specifically using Google Apps Premier for our own domain.