Instructions
Go to your Windows Live account and sign in with your Windows Live ID that is linked to your Xbox Live account.
Click "Options." This link is in the upper right side of the web page. Then click "More Options."
Click "View and edit your Personal Information." This link is under the section "Manage your account."
Click "Registered Information." This will open a page with a summary of your account.
Change your date of birth listed under the section "Birth date." Click "Save" and your age will be corrected on your Windows Live account. This will change the age on your Xbox Live account and your Xbox profile.