I'm not sure what you mean by it didn't count, and I don't know if being an AU will always mean you're approved for a CC right away. However, the child's credit history should be established on the date that CC was opened. If you opened a CC in 2010, added him in 2015 as an AU, that CC should still show up on his credit report as having been established in 2010. Was it a card that was used frequently? If it was a card that wasn't used much, there wouldn't have been that much to report to the credit bureaus in terms of on-time payments, etc.
Does he have his own bank account? DD stated getting offers from her bank about 6 months after she opened her own account. She got a better offer/card(reward) then I did when I got my own credit card through our bank. LoL Now she has even better credit than her dad and I.DH has had the card for quite a while, and added DS when he went off to school (2019). - The bill still comes to DH, though. It's not DH's primary card, but he uses it occasionally, and DS quite a bit for small things. It is always paid right off.
The person DH talked to about getting DS his own said he needed something in his own name. - I definitely think we're applying through the wrong company!
Does he have his own bank account? DD stated getting offers from her bank about 6 months after she opened her own account. She got a better offer/card(reward) then I did when I got my own credit card through our bank. LoL Now she has even better credit than her dad and I.