Addition lowered your average age of accounts, as it is a brand new account, so take a look at that opposing effect, which most probably did not offset the gain you received by increasing your overall CL.
Probably a net gain, but not assured....
Additition as an AU might also cause some issues if and when you app for your own new credit, as your score is now based, at least in part, on the credit history of another.
If you app for credit that involves a manual review of your credit report, and the creditor wishes to determine your score without the presence of the AU account, they have no way of doing so. Just be aware of that potential.
The hit to your AAoA will fade fast, then you it will begin to grow again.