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What is the Age of Your Oldest Account? This "add me as an AU" technique is fine, but it is really only a good idea when all of the following are true:
(1) The age of the card to which you are added is much older than the age of your oldest account
(2) The card can be kept at an ultralow balance (zero is best)
(3) The card has no lates associated with it.
We know from you that #3 is true but don't know yet about the first two criteria.
Bear in mind with #1 that Age of Oldest Account is not the same as Average Age of Accounts.
Finally, even if all three are true, there is a chance that FICO 8 might ignore it anyway after it appears on your reports. There's an easy way to test whether that is true, however, so I would not worry about that.
I think the second piece of criteria, utilization of the proposed AU card, is often the overlooked piece. Some people add an AU account to inflate credit age depth, but don't realize they're adding an account that's at (say) 50%-60% utilization, so the knock from the utilization erodes the gains from the greater age of accounts.
BBS is right...make sure you find out about the utilization %.
I did the same idea as you...was added to an old account, low % and got a nice 15pt bump...I am sure I will get more once they make some bulk payments.