It seems to me that a lot of old threads are being revived by people lately. I was just wondering whether these should be reported as spam if it's no longer relevant, like relationship advice for a thread started two years ago or a computer issue that was posted six months ago.

Absolutely, if the reply fails to meet the 'standards', adds nothing to the discussion, or otherwise violates the guidelines, please hit the Report button and request that a Moderator review the posting.
This is a problem which is being reviewed. Some forum software (AEF Forums for instance) has the capability to have a pop-up window when a Member is replying to a 'stale-dated' Topic.
It doesn't stop the posting, simply warns that this is an old Topic and do you really want to proceed. It might be available as an IPB add-on. we are looking at the options.

On the AEF boards, the 'age' can be adjusted in the Forum Admin Area, so the 'stale-date' is configurable.

Isn't there a "pruning" option for old threads in IPB? I never used the function because they were small forums and there weren't a lot of people so it was much easier to moderate, but I seem to recall that you could prune topics that are x amount of days old.