From usairways.com:
Retroactive credit
•You can request credit for past flights on US Airways, oneworld alliance member airlines and other eligible partner airlines.*
•You can only request credit for American Airlines flights flown on or after January 7, 2014.
•All requests must be made within 6 months of the date you completed travel.
•New members can get credit for transactions completed up to 6 months before they enroll in Dividend Miles.
See also:
http://www.flyertalk.com/forum/us-ai...redits-us.html