Unable to update date of birth associated with an Apple ID: "... could not be changed because of a server error"

You can't change the date of birth or family relationship of an Apple ID with a calculated age of less than 13 years old. Otherwise it's supposed to be possible to change the date of birth associated with an Apple ID. I've done it before (for good reasons).

Recently I decided to get #2 child an Apple credit card. Since banks take birth dates seriously I decided I needed to correct his before applying. It didn't work!

This is what his birthday looks like on his Apple ID web page (same as in his iOS devices):

When I edit it to 4/2/1999 I get this message as expected:

The next step is an email sent to the me.com (we're old customers, pre-icloud) associated with my Apple ID:

Can you see what's weird in this email? It says the change will convert his account to a child account. But the current date makes him 17, the correction makes him 22. The email should say it's being converted to an adult account.

When I click on "approve request" I get:

Cannot change date of birth.

The date of birth for ___ could not be changed because of a server error. Try again.

I started an Apple Chat Support ticket on this a week ago. After the usual back and forth I was escalated twice and ended up with a senior advisor (not sure of his title).  After a bit of work and repetition I was told a ticket had been sent to engineering to fix the birth date and I'd hear back the next day.

It's been a week. I've not heard back. I'll try a chat again tomorrow. Fortunately I have my original ticket number.

I suspect his account entry is somehow corrupted and possibly mixed with another account. Otherwise I wonder if this is a side-effect of Apple introducing family credit cards; maybe once I got an Apple credit card all the family birth dates were locked as a side-effect.

It's not a common problem though, I found one similar report from someone trying to convert an adult to child account.

This is going to be a painful slog with no certainty of success.

UPDATE 7/13/2021: After hearing nothing back I called again and was again escalated from chat support to phone support to "senior advisor". Senior advisor said engineering had responded and said it was not actually possible to change date of birth on a child account. I believe that is wrong, it contradicts Apple's documentation saying it should be possible to change for over 13. I have asked that it be reescalated and attach the tech support ref: HT204164.

I reserved a time to call back in 3 days and was told I'd get a link to enable a direct call to a senior advisor.

PS. Related Apple Discussion.

Update 7/24/2021: Support didn't call back on the date they'd promised. There's still a server error. As noted above Support seemed unaware of the contents of HT204164. 

I'm going to have to give up on fixing this for now. I'll try again in a few months, maybe by then Apple will know how to fix the problem. I suspect it's something in their database design that will require serious work to fix. I also suspect their support team is dealing with post-COVID stress syndrome.

Update 1/1/2023: Today I was able to change his birth date without issues. Based on the original birth date he would be 19yo now, so an adult.