Let's say I have a Citi opened in 1980. I add you as an AU. Your open date becomes 1980, with the balance, CL, payment history, TL comments, and everything else.
Most all CCs do this.
Now let's say you are on the Citi, I cancel the CC next month. The TL remains at 1980's date. You open a new CC tomorrow and it will show 2012, not 1980. Nobody backdates to a date outside of the added TL. Only Amex used membership dates and as pointed out, that's in jeopardy.
Thank you for the perfectly clear example.
I'll see what I can do with that.
My parents have other credit card accounts.
I'll try to find the cleanest one and, just as a precaution, will ask the company itself about its policies before proceeding to encourage my parents to add me as an authorized user.
Once more, I am grateful for the answers.
Keep up the excellent work!