If you have a worthwhile answer, then go ahead and post it even if it's an old thread. It's not against the rules. May be, you have a new answer to an old problem.
Are you concerned that an answer to an old thread would not have enough visibility? When an old thread gents a new answer, it gets bumped to the top of the
active stream of threads. Folks will get a chance to vote on your answer.
Are you concerned that bad answers can be posted into old threads? That's taken care of. Late answers are a subject to additional moderation. There is a separate review queue for late answers. You will see these review queues when you accumulate enough reputation.