From
http://www.aa.com/i18n/AAdvantage/termsConditions.jsp
Originally Posted by AAdvantage T&C
You may request mileage credit for past, eligible transactions up to 12 months from the transaction date. Transactions prior to your enrollment date are not eligible for mileage credit.
so for the flights prior to the enrolment date, no credit. If you managed to get the AA flights prior to joining date credited, you were lucky