Or have "value" exponentially decay over time (e.g. continuously & adding up to 50% decay per year. Which is equivalent to the value of a vote being 2^(year the vote is cast).)
If upvote===1 point and downvote===-1 point old posts would get a head start (from being in the top lists) but it wouldnt be that hard for a competing link. Bad links would quite quickly get low rankings though.
An important thing to note is that this isn't a social news site (as reddit or HN) but a social link sharing site, and thusly doesnt need to have fresh link at the top, but only best links.