Apple will drop support for logging into iTunes using your AOL username and password at the end of March 2015. Apple originally partnered with AOL for logging into iTunes in late 2003, and used to give users the option to log in either with their email address and password or their AOL credentials. Will still be able to use your AOL email address to register for iTunes.
Apple will pull support beginning March 31, and users must convert their username and password to an Apple ID before that time, according to the company's support article:
You must convert your AOL Username to an Apple ID in order to maintain access to the stores and to content you purchased previously.
You can find Apple's support page with the instructions for converting your AOL username and password into an Apple ID at the link below.