You can easily hide posts older than a given time relative to present by using Hide Old Posts plugin. The plugin is not yet updated to WordPress 3.5.1 but due to its simple code it works with most recent version of WordPress 3.5. Install and activate it, after activation visit your admin area Dashboard -> Settings -> Hide Old Posts page and set the following options.
Now specify the value valid according to
strtotime to set how old posts shall be shown to other users, Default is “-2 years”. It provides you option to Hide posts only, if left unchecked it hides other post types like old pages, attachments etc aswell. You can make it show singular content and manage user capability to see all posts.