If your parents add you as an AU to an active account of theirs, your account will have the same open date as your parents.
Prior to 2011, you could then create your own account with your parents MSD date because the account you are an AU on has an older MSD date.
Post 2011, Amex will not backdate to your parents MSD date on any account of yours other than the AU account. Any other account you create will have an MSD date of the year your first started your relationship with Amex.
To illustrate, let's use this as an example. Your parents have an Amex Green with an MSD date of 1980. In 2013 they add you as an AU. In 2015, you open an Amex EDP. Your account open date on the Amex Green will show 1980. Your MSD date on your EDP will show as 2013.
Let's say your older brother was added as an AU to your parents Amex Green in 2009. He then opened an Amex Platinum in 2010. Your older brother will have an MSD date of 1980 on both cards and any new cards he opens.
Simply put, Amex changed the way they backdate. You can only backdate to the original date you started your relationship with Amex. However, like with any other AU account, on the specific account you are added as an AU, you will inherit that accounts history.
Finally, if your parents closed all their Amex accounts, they can't add you as an AU without first opening a new account. You can't be added as an AU to a closed account. Also, adding your mom as an AU to your account won't change your MSD.
though it would've been nice to get a 1988 MSD!
lol