This nightmare started in Jan 2010 and I am hoping 2 years later there is a solution.
I am an adult and have been for many years. In 2009 I added my CC and promoted my account to "Adult" and it worked fine.
Then in Jan 2010 I changed GT and Live ID and since then its reverted back to Child Status. I have a Promote Account button showing up again and a "Leave Family" option. I am not a part of any family so I cannot leave one. I tried adding my CC again and it ended up in an error.
I called XBL support in Feb last year and they said that my account was "Adult" (Successfully Promoted) and they were aware of this problem people were experiencing. I was asked to wait a few months as it would automatically be resolved. It never way.
The problem with this Child account nonsense is I am unable to access some content and its a pain. Is there a fix for this yet?