Last month Apple issued a support document stating that as of June 30, 2014, it would stop support for AIM users with mac.com or me.com IDs. Apple noted that by upgrading to OS X 10.7.2 or later, however, users could continue to use their mac.com or me.com accounts.
According to a post by AppAdvice, Apple decided to discontinue support for ALL Mac OS X users that continued to use mac.com or me.com accounts with the built-in Messages app and other third-party applications. While we’ve had several instances of users who are running Mavericks being unable to login with me.com or mac.com in Messages, some users running 10.7.5 are reporting that they have had no change in AIM service and are continuing to login to iChat with legacy me.com and mac.com accounts.
If you can’t login to AIM via Messages or iChat, Apple’s support document gives instructions on how to create a new AIM ID to use with whatever version of OS X you have.