The way I find the AOL IMAP service to work, is that you can choose to store Sent, Drafts, and Trash on the server, and you can instruct Mail to use those folders like any normal IMAP.
However, they all fall under AOL's retention policy of only 27 days (as does the Inbox). Thus, to stay on the server longer than 27 days, the messages must be copied or moved to folders kept within the Saved folder AOL provides on the server. See:
http://members.aol.com/adamkb/aol/mailfaq/imap/
See the section about AOL's IMAP folder structure in the article. There is a link there that can also be followed once you type in your screen name and password, to further explain folder policy on AOL's own site. Retention in the Saved folder depends upon total size, I believe, and not time.
Ernie