I'd be very hesitant to remove old content. I used to do SEO for some very big (and very old) newspaper sites, and we had the same debates. I always encouraged them not to delete old content. I didn't always win those battles (it's hard when an executive is saying, but we can save $x a month by reducing storage), and I couldn't quite quantify the value of keeping it in the same way.
If you do keep it, one thing you can do is make it clear that the article is old (put 'from the archives' on all pages older than a year, for example), and then have a separate navigation path that makes it clear that these are old articles you're searching / browsing.