Fellas, I just had a long talk w/ Amex Credit Bureau and received an education on how the backdating actually works.
The whole thing with getting added as an AU to an older account used to work. They stopped doing that in Nov of 2011. If you were added as an AU before that date, then your account will backdate to the primary. If you were added after Nov 2011, it does not work. You will have your own account opened date.
However, backdating is very helpful still in one way. ALL of your Amex personal cards will backdate to the date of your membership. I had a CS rep who swore up and down that it ain't so, but the Credit Bureau Dept at Amex told me that CS doesn't know what they talkin' 'bout. (Shocking, I know.) In my case, I have a 22 year old Green Card. This month I opened SPG and Delta Gold personal cards and they will both report as having been opened in my membership year of 1992.
If you are interested in a technicality, I will tell you that the MONTH of the newer account will show as the actual month that it was opened. The YEAR however will show as the membership year based on the oldest card.