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.