I encourage users to revive old threads if they have something to add to the discussion. A large number of threads on my board aren't really date sensitive, such as threads dedicated to a particular game/series, or asking a user for their opinion or experience about something. The only threads I wouldn't want to see bumped would be those that are for a particular time, such as bumping a birthday thread from months ago, but most of those types of threads just get locked when there's no need for further comments.
It's fine as long as they don't do it to "bump" the topic. In fact, I'd rather have one long and active topic than several topics around the same subject. So yeah, I'd encourage people to search for older topics around the same subject or question before they post a new topic. I occasionally also merge topics.
If they have a question related to the topic or any information they want to add to me I do not see anything wrong with that because this will prevent creating of similar topics.. I will advise them to comment on it as long as it does not violate the rules of the community and only when they are really interested to get ideas or information.
I dream of the day when I have discussions that are 1 to 5 years old !
And I will look with kindness on the first member who will bring one back to life !