Once you lock in your MSD (Member Since Date) the year of subsequent cc's will reflect your MSD. Additional cc's*, will reflect the month of THAT card and the year of your MSD.
If you are a primary cardholder, your CR's will be backdated as well. Amex recently changed their policy to exclude the CR backdating for ACMs - when you become a primary, your CR's would be backdated for YOUR new cc.
In in this case, adding DD now will lock in her MSD for 2014. When she applies for her own Amex in say,5 years her date was locked in for 2014.
*personal cards - business cards it's a YMMV
