Amex can be kind of funny. You can have a sucession of cards with them and they will still report your account age back to the oldest card you had. When my wife got her US residency and we added her social security number to her account as an authorized user on my card, they showed her as having an account since 1995. However, since you have two different products (a pure Amex card and a co-branded card) that might not happen for you. There's probably not much you can do, but I wouldn't worry too much about it. It's not going to kill your credit score, at worst it should be a slight ding.