I've got a site with an events section on it, and two months after the event passes i put a "noindex" on it so it drops out of google's index.
i also allow people to search by date, and i always "noindex" those date pages once they pass too.
i always thought this was the best way of doing it, because having a load of old events in the index is pretty pointless. but my site has been going for a number of years now, and the number of "noindex" pages is huge.
the alternative is to just delete these pages altogether, but i dont really want to do that in case i lose a load of backlinks.
do you think that google takes into account the percentage of a site that is "no indexed", when it tries to rank it? because you're basically telling google that you don't want people to see these things. if the majority of your site is out of bounds, then maybe they'll take a dim view of it
there is one other alternative, of course... and that is to just allow google to index the whole lot, even though they are old. it might even bring in a bit of extra traffic.