The question that matters is how PenFed will report the date opened. Suppose your father opened those two cards in Jan 2000 and he added you in Jan 2018. Will the Date Opened on your credit reports read 01/2018 or 01/2000? (Some CC issuers do it one way and some do it another.)
There's no way to know until the AU cards appear on your credit reports. What a PenFed customer service rep says is pretty valueless since CSR's almost never know anything about credit reports.
If I were you I would sign up for Credit Karma if you haven't already. Then just pull your reports once a week and see whether the accounts have appeared (and if so how the Date Opened reads).
