Xbox Live adult/child account issues

You can't change a child account into an adult account once registered for xbox live. You can modify the DOB under your MS account's page yet that will not upgrade a child account into an adult account for the live service.

I found this, hope it helps.

Promote a child account to an adult account in Xbox

Xbox LIVE determines whether an account is a child account or an adult account based on the age of the account owner. Account owners who are younger than 18 are given child accounts in the United States. The legal adult age varies by country and region (for
example, the adult age in Korea is 20).

Child accounts are tied to an adult's Microsoft account. The adult account can control Family Settings and restrict some features of Xbox LIVE.

After you reach the age of adulthood (which varies by country and region), you can promote a child account to an adult account. This promotion removes the Family Settings and Xbox LIVE restrictions.

To promote a child account to an adult account

Sign in to Xbox LIVE by using the account that you want to change.

Go to Settings, and then select Account.

Select Promote Child Account.

Note The age of an account owner is calculated by using the date of birth provided when the account was created. This date cannot be changed. If you entered an incorrect date of birth when you created the account, your only option is to create a new account
using the correct date of birth. This option is available only if the date of birth for the account indicates that you have reached the age of adulthood (18 in the United States). The legal adult age varies by country and region.

Follow the on-screen instructions to promote the account.

You will need an XBOX 360 to Promote a Child Account

http://support.xbox.com/en-US/xbox-live/online-privacy-and-safety/online-safety

Promote a child account to an adult account

Xbox LIVE determines whether an account is a child account or an adult account based on the age of the account owner. Account owners who are younger than 18 are given child accounts in the United States. The legal adult age varies by country and region (for
example, the adult age in Korea is 20).

Child accounts are tied to an adult's Windows Live ID. The adult account can control Family Settings and restrict some features of Xbox LIVE.

After you reach the age of adulthood (which varies by country and region), you can promote a child account to an adult account. This promotion removes the Family Settings and Xbox LIVE restrictions.

To promote a child account to an adult account

• 1.Sign in to Xbox LIVE by using the account that you want to change.
• 2.Go to My Xbox, and then select your profile.
• 3.Select Manage Account.
• 4.Select Promote Child Account.

Note This option is available only if the date of birth for the account indicates that you have reached the age of adulthood (18 in the United States). The legal adult age varies by country and region. The age of an account owner is calculated by using the
date of birth provided when the account was created. This date cannot be changed. If you entered an incorrect date of birth when you created the account, your only option is to create a new account using the correct date of birth.

• 5.Follow the on-screen instructions to promote the account.

Xbox Live determines whether an account is a child account or an adult account based on the age of the account owner. Child accounts are tied to an adult's Microsoft account.

After you reach the age of adulthood (which varies by country and region), you can promote a child account to an adult account. This promotion can be done only from the parent's account

Xbox LIVE determines whether an account is a child account or an adult account based on the age of the account owner. Account owners who are younger than 18 are given child accounts in the United States. The legal adult age varies by country and region (for
example, the adult age in Korea is 20).

Child accounts are tied to an adult's Windows Live ID. The adult account can control Family Settings and restrict some features of Xbox LIVE.

After you reach the age of adulthood (which varies by country and region), you can promote a child account to an adult account. This promotion removes the Family Settings and Xbox LIVE restrictions.

To promote a child account to an adult account

1.Sign in to Xbox LIVE by using the account that you want to change.

2.Go to My Xbox and then select your profile.

3.Select Manage Account.

4.Select Promote Child Account.

Note This option is available only if the date of birth for the account indicates that you have reached the age of adulthood (18 in the United Kingdom). The legal adult age varies by country and region. The age of an account owner is calculated by using
the date of birth provided when the account was created. This date cannot be changed. If you entered an incorrect date of birth when you created the account, your only option is to create a new account using the correct date of birth.

5.Follow the on-screen instructions to promote the account.