Apple has added a support document to its website stating that customers who use an AOL username to sign into the iTunes Store, App Store or iBooks Store will be unable to continue doing so starting March 31 (via 9to5Mac). Apple customers must convert their AOL username to an Apple ID account in order to maintain access to the storefronts following the deadline.
"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."
Apple will no longer provide support for AOL usernames that are not converted following March 31. The transition does not affect any iTunes purchases made with the AOL username or any other AOL services that may be associated with the account. To begin the transition process, users must sign into iTunes with an AOL username and follow the on-screen instructions.