When trying to sign-up as an Age Insider, if you are unable to start the registration survey and receive a blank page after signing in with your Microsoft Account, please take a look at the URL of the blank page. If the URL is ending with "error_msg=no_xbl" then please follow this step:
🛠 Sign Up for an Xbox Live Account (account.xbox.com)
In order to register as an Age Insider, your Microsoft Account needs to be linked to an Xbox Live account. Creating an Xbox Live account is free with any valid email address.