I am having exactly the same issue. When I try sign in as him on xbox.com it says he needs a parent to sign in, I sign in and it prompts me to allow him access to set up an account (which of course he's had for over a year), when I accept this it says the address and dob is missing, but we can't get into the account to edit it.
Maybe there is a wider issue, given its affected more than one person in exactly the same way at roughly the same time!
As its Christmas and he is desperate for access to his mates with his new games I have added a 7 day free trial on my account. Has anybody got any answers yet please. This has been going on over a week.