Starting March 31, 2015, AOL will no longer allow customers to use their AOL Username (also known as an AOL Screen Name) to sign in to the iTunes Store, App Store, or iBooks Store. You must convert your AOL Username to an Apple ID in order to maintain access to the stores and to content you purchased previously.
Convert your account
To convert your AOL Username to an Apple ID, sign in to iTunes on a Mac or PC with your AOL Username. Then follow the on-screen instructions.
Your new Apple ID will maintain your access to the iTunes Store, App Store, and iBooks Store, as well as all of the content you bought using your AOL Username. You can also use your Apple ID with iCloud and other Apple services.
Apple notes that it won't be able to provide support for AOL Username accounts that aren't converted by March 31st.
More details in the Apple support document linked below...
Read More [via MacLife] [via Ari]