Recently, I opened up a business Amex with a couple of AUs. In the past, each AU card would have the same "member since" date as mine. However, this time, while my card has my original "member since" date, both AU cards have "member since 12," eventhough each of them have Amex cards with member since the 90s.
The last time I added an AU, at least the AU had a "member since" their original date, eventhough it wasn't backdated to mine.
I applied for my first Amex, a BCP, a couple of weeks ago.
I was added as an AU on my father's acct decades ago.
My BCP showed up with a member date consistent with the year I was added as an AU.
Amex has informed me that it should report to bureau's to that date as well.
I will update when it reports and what date it reports as.