I have a question that may seem dumb to some of you, but please bear with me. I recently had my father add me as an AU on his Amex platinum that's been open since 1997. It just showed up on my experian credit report as having been opened this month. I had thought that if you're added to an old card, it's shows up on the credit report that you opened it when the account holder did. Can someone explain this please?