For instance a 2014, 2015 sale. Is it better to update that page, or create a new one each year and leave the old ones up?
It has been widely accepted by the SEO community that both Google and Bing reward for pages that receive minor or major updates to the page. It would not make sense for search engines not to reward sites without some form of influence. Information can naturally overtime and refreshs most often increase user experience which we know is something Google particularly cares about the most or at least thats what they keep feeding us with.
Google owns thousands of patents and some of them allow us to speculate how Google's algorithm works. Here's a patent that Google owns about scoring a page that changes over time:
A system may determine a measure of how a content of a document changes over time, generate a score for the document based, at least in part, on the measure of how the content of the document changes over time, and rank the document with regard to at least one other document based, at least in part, on the score.
But as Matt Cutts often says "just because Google has patent X doesn't mean they use patent X", however you should be able to find various articles supporting what I've said to be somewhat true. However much Google or Bing rewards for such updates is left to speculation because they never reveal the extent of how much weight is put into each factor of there complex algorithm.
I recommend that you check out:
First I'd check the traffic to those older product pages. If it's decent but you don't sell those products you should consider:
- updating the page saying the product is Eol (end of life), and adding an obvious link to the newer product or
- redirecting those pages to the newer product to pass along the SEO goodness earned.
One consideration is to never leave the visitor hanging wondering where to go next. They landed on those old product pages for a reason (which you should research how/why), so give them a clear next step action. You don't want to take the risk they give up and leave your site without knowing there are new products.
In many cases there are legal reasons (support, SoX) for retaining older product pages so check first before removing them. You could also move them to a different menu structure so it's clear they are indeed older (no longer sold?) products.