How to make sure macports only upgrades to ports that are at least a month old?
You could potentially monitor the SVN repo that all the Portfiles are distributed in to determine when last each port was changed.

Take a look here: http://guide.macports.org/#development.local-repositories, and set up a local copy of the macports trunk. Once in a while go see what revision number (#####) was committed about a month ago, and svn upgrade -r #####

Make sure you remove the default macports source, though:

#rsync://rsync.macports.org/release/ports [default]


Clearly a different approach to where you were heading with the port command, but it would work.