Xbox live subscription

I have a question regarding what would be best . We have 3 xboxs in my home , and have a family account with each child having there own profile. The oldest child (14 years old) is going to get his own subscription to Xbox ultimate. We have always made any of the kids purchase games under my account as the adult account so they could all enjoy. My question is would it be better to add the ultimate to his child account under my family account or would we need to make him another separate account to add the additional ultimate? Another of my questions is if we do add it under my adult account when he becomes an adult dose his account automatically turn into his own separate adult account, or is that an option after a certain age or is that child account linked to my account always connected to my family account? Were trying to not make him lose all his games but if at some point he will have to then we'd rather do it now so he has time to build his game library up again before he is an adult and moves out .... I hope this makes sense .... any advice or information would greatly be appreciated. Thank you

:)