As we've probably said before, how we (users, site visitors, and moderators) feel about a reply to an old thread depends on the apparent reason for the post.
If a user is answering an unresolved question from long ago, why not? If the thread was quiet but is once again relevant, posting is perfectly appropriate and may in fact be preferable to a new thread on the same topic, so that the prior history and context are available. Sometimes a thread is relevant by season. I know a few threads that are woken up once a year for new conversations.
But... if a user was carelessly commenting in a no-longer-relevant thread that happened to show up in a search result, waking up the old thread is probably doing no good for anybody. If there was a more recent thread about the same topic, their post should have been there instead.
Sometimes shills and spammers who we haven't yet caught will bump old threads because they are searching for keywords and posting in any thread that shows up in the results, without regard to thread age. We'd like to know about these so we can remove the posts AND the users.
There's no forum rule against bumping old threads, since all these factors come into play and it takes common sense and judgment calls. If you think a post in an old thread is a sign of a problem, let the moderators know.