Originally Posted by
SPN Lifer
AmEx issues a different account number for the authorized user (AU). Chase and Capital One do not.
The issuance of a separate account number would seem to be the determinative factor in reporting when that account was opened.

At least for Venture X accounts, Capital One does issue Authorized User cards with unique numbers. See
HERE
Even if the embossed card numbers are the same, the issuer can tell them apart, and could report a different account opening date if they chose.
I’m looking at our Cap1 Venture X cards and all have different numbers: 1 primary, 3 AUs.
I’m going to upgrade the AU gold Amex cards to AU plats in a few months. I hope the numbers remain the same.