Xbox Live accounts are managed by Microsoft which means you can use your Live.com account to change your personal account settings, including the age setting of your Xbox Live account. For now, you still can't change the age on Xbox Live using an Xbox console; however, you can still manage it through your Live.com account settings.
Step
Step 1. Visit the official Microsoft Live website at login.live.com
Step 2. Enter your email address and password, then click Sign In. "
Step 3. Click on your name located in the upper right corner of the Live page, and select Account Settings. "
Step 4. Click "Edit personal info" which is located below the date of birth
Step 5. Enter the new date of birth, then click Save. Your age will also be updated on all Microsoft accounts you have, including Xbox Live.