Apple has supported chatting over the AIM network with iChat using an Apple ID ending in .mac or .me since OS X 10.5. (Note: the iChat app was renamed Messages with OS X 10.8 and later).

Effective May 17th 2014, Apple will require OS X 10.7 or later to chat over the AIM network using a .mac or .me Apple ID. This means if you are using iChat with OS X 10.5 or 10.6, and you have an account ending in .mac or .me, you will need to create a new ID at AOL’s website if you wish to continue to chat over the AIM network.