Apple has different age restrictions in different regions, but most of them trigger on/off based on the age of 13.
If the birthday of a child account is entered incorrectly identifying the account as under 13, notable features (test flight, apple pay, and many others) on iPhone will not be available.
Is there a way to correctly update a child's birthday, which moves them from a child account, to a teen account, and activate these features?
After much time, I now understand more about the changing of a birthdate on a child Apple ID account.
In my region (US and UK) there are bands which define child, teen, and adult. They are....
Basically, you can't change a birthday from one band to another. Once a child, always a child, until the system thinks the birthday rolls into the next band.
So, once a child account is associated with a birthday under 13, it can not be changed outside the band of 0-12. But can be changed to 12 years, 364 days backwards from today.
So here is the fix: Change the birthday to today - 13 years + 1 day.
Example
Today is August 10th, 2019, so you would use August 11, 2006. Then, the next day the account will "naturally" roll over to 13, and at that point you can change the birthday to anything in the teen band of 13-18.
I did have to also use the parental approval to approve the birthday change. So my steps were ...
