This passage will guide how to retrieve your parental email address. If you just forget the password, you may reset the password.
I have posted on Microsoft Answers, Xbox Forum, contacted with online agent and even make a call to Xbox. (Thanks they understand my Asian English) No one can solve it. Then, I have a solution.
First, you need to go to this page where says parental approval required. You need to enter parental email address and password. The problem is that you or your parents forget the email address.
By carefully scrutinizing the URL, I find parental email address is included at the end of the URL! Microsoft, you are so smart. Merely change "%40" to "@" and you get the email address.
Once you have the parental email address, you can reset the password or Microsoft agents can recover it.