If the new question doesn't differ from the old question, it should be closed as duplicate.
The fact the old question doesn't receive new updated answers doesn't mean a new question should be asked: An old question that has not been closed can always be answered. I answered some old questions, reporting what the actual situation is, and my answer was up-voted and accepted.
The difference is that, eventually, new answers for the old questions are not accepted from who asked the question, but future readers will read the new information the same. An accepted answer is just the answer that helped the OP more than the other answers; it doesn't mean it is necessarily the most correct answer. In fact, I have seen accepted answers with a negative score.
If users want to have updated answers for a question asked from somebody else, they can offer a bounty for the question. Alternatively, they should ask a new question making clear in what the new question is different from the existing one.
Take the following two questions as example. (I am sorry: it is not the best example I could think of, but I should make things clearer.)
- What Drupal module can I use for [description of the task to achieve]?
- What Drupal module can I use for [description of the task to achieve]? I have looked at [link to the previous question], but I cannot use any of the modules suggested in the answers there because [explanation].
The second question says the OP cannot use the modules suggested for the other question, which means it has more restrictive requirements about the modules to use; it is not simply re-asking the same question, since the answers given to the other question are not acceptable.
If a question is asking the same thing asked by another question, and it is not putting restrictions about the acceptable answers, then it should be closed as duplicate, independently from when the previous question was asked.
Since it is not anymore possible to close a question as duplicate of a question without any up-voted/accepted answer, the way to close duplicates changes.
In the case the older question doesn't have answers, and the new one has an up-voted answer, it is only possible to close the old one as duplicate of the new one. The alternative would be writing an answer for the old one, and wait it gets up-voted.
If the older question has an answer that is not up-voted, you could up-vote it (if it is worth voting it), and then voting to close the newer one as duplicate of the old one.
If then both the questions has up-voted/accepted answers, the rest of the answer is still valid. I would also add that I would consider other factors, before deciding which question to close. For example, if the old question was asked when Drupal 5 was still supported, and the new question is asked when Drupal 5 is not anymore supported, Drupal 7 is the official release, and Drupal 8 is under development, then probably I would opt to close the old Drupal 5 question, and keep open the other question which would gather answers for the newer versions.