The age of a question shouldn't play a role in closing of duplicates. If everything worked perfectly, we would be able to close duplicates as they arise so new answers would add content to the original question. However, there will be times this doesn't occur. When these situation do arise, the better written question and answers should be the lead the post not the older one. Even though a younger post may be the lead, people who visit either of the post will see on the right linked post or on the top for the duplicate the duplicate link. That is, if they are so inclined, they can explore similar questions and answer.
Then there is the option of merging the post as Mad Scientist suggested in the comments. This is a fine option as well as long as nothing is lost in translation with the question body. If one question body has a small detail which the answers were addressing that the other post did not contain, the flow of the answers may seem strange unless a note is added to the OP of the merged threads specifying this anomaly in answers.
Additionally, judging a question by vote count is very fickle. Many factors can play into a question receiving a high vote count. One factor I have noticed on other SE is that questions that were around when a site first entered beta tend be scored very well since the site has that new car smell. For instance, consider this question from 2008 on SO. This question would probably be downvoted and closed today in seconds but it has 212 upvotes. Moreover, the first 6 questions to SO have a vote count greater than 100. If you want to see bio, here are the questions order by age. The earlier questions received many upvotes compared to the voting now. Yes some question will come in later and be so tantalizing that they will garner a lot of action but it gets harder as the site ages. That is, vote account isn't the best metric to determine post quality if age between post is great.