Sometimes a thread is not really solved and a poster can post the solution (as far as this applies in this type of forums). So in my opinion a thread should never be closed for this reason.
If something is implemented, the 'timer' should be based on the date of the last post and not on the date of the opening post (so the nifty fifties don't have to worry as long as they keep on posting
)
Some options as I see them for old threads (last post older than one month)
- disable quick reply and replace with warning (as shown by Adam)
- a warning in the thread title (using another colour or a message), visible in the specific forum as well as when the thread is opened.
- a page redirect; default the hyperlink takes one to a warning page and automatically redirect after 20 seconds to the real page
- any combination
WimS