If added AAoA is the only or the primary reason for adding her as an AU, you might want to ponder on that unless her score is really low and she is beginning rebuilding.
Age of accunts is not a major factor in credit scoring.
As soon as you add any account of yours to her credit report, that will automatically make her credit score not representation of her personal risk of repayment.
A credtor, upon doing a manual review and seeing presence of an AU, may thus choose to either discount or ignore her score in their evaluation.
Adding an AU is good when rebuilding, where the person is appling for low amounts of credit wherein the credtiors dont normally go to the time and expense to do a manual review. However if she will be apping for normal or higher levels of credit thst traditionally involves a manual review, having an AU could be a detriment even if it produces an artificial score boost.
