Șerban

On every single post, older than 1 year (365 days), you may add a custom formatted text, in order to let your readers 
know the information included might be outdated.

Old Post Warning

This plugin will display a custom notification on every single post older than 1 year (365 days).

Ben Meredith

WP Old Post Date Remover removes the distractingly old date from otherwise perfectly good evergreen content on your WordPress blog.
This plugin does not deal with the date on SERPs or archive pages. Single posts only.
You spent the time creating great content, and now readers and potential customers are getting sidetracked by the fact that it’s 13 months old.
Install the WP Old Post Date Remover plugin, activate it, and pick the number of days you’d like to show the dates on your posts.
NOTE: if your theme does not place the date within its own span class or div class, (or does that annoying thing where they put the word “on” outside of the date’s span or div) this plugin will not work, out of the box. But with a few extra lines of code, we can get you working in no time. Message me in the forums.
This plugin requires special setup for use alongside some caching setups. I’m happy to help get that set up for you. Message me by starting a thread in the forums.
Please rate the plugin, and don’t be afraid of the donate button! I am active in the support forums if you need assistance. Thanks for checking us out!

WP Old Post Date Remover

Removes the date stamp from older posts (you choose how old), while leaving the date stamp on newer posts. Ideal for blogs with evergreen content.

Old Post Warning vs WP Old Post Date Remover