I'd like to see the age of a file (time since last modification) in some human-friendly units (bonus points for things like "yesterday", "2 days ago", "3 years ago", although just a number of days would be sufficient).

Is there a shell one-liner that's simple enough to memorize and type on demand? Is there a tool (packaged for Debian/Ubuntu)? Do I write a custom shell script to do some arithmetic and install it on all the servers I have?