If you want to re-activate a topic because you think it is an interesting discussion then there probably is no “too old”.
But if you want to answer a question for a user then you should make sure that the user still is active. So if he did not log in for months then it’s probably too old.
If you just want to tell an admin/mod that the topic should be deleted because it’s against the rules or just spam then you should use the Flag Post - function as I mentioned before.