I've never seen anything like this, although there are date-related functions that do things like remove the post date on older posts to avoid having them appear dated. This is particularly true with Google displaying post date right in the results. To add the noindex to older posts, I think you'd need to add a conditional statement in the header. Good caching should minimize any minimal performance hit.
Do these older pages generate no traffic that could be useful to the site? Perhaps you could leave them in the index and beef up cross-linking to current content. That way, if someone hit the "obsolete" page they would have some relevant choices to reach your newest content.
I went with the conditional statement in header.php
Those pages generate very few traffic and really are outdated (pasts events). I want to keep them for references and because some of them have inbound links but there is really no need for them to appear in the search engines index anymore. And they represent quite a big percentage of the total content.
I'll keep them but warn the reader of the outdated content and prominently display related current events to help them find what they are looking for. I'll see how it goes.