If I am added to someones amex card as an authorized user and he dosnt have a good credit score will that effect my credit score?
I'm adding my self to try to lengthen my credit history lifetime with his older account
A) Being added as an AU on an amex card is useless in terms of using the account age to boost your length of credit history. Amex AUs report as opened from time AU was added, not from when card was opened.
B) The credit score of the primary cardholder doesn't matter, but the history of the account you're being added to and going onto your report does. (Incidentally, since the history of a primary amex card doesn't go on the AUs report, it shouldn't matter tremendously if there's a negative in the past, IINM.)